Some countries are thorough in their use of the tentative list. Tendentially, each of the TWHS is proposed before it is either inscribed or retracted. These countries seem to be the exception. Other countries revise their T-lists more frequently without necessarily going through the nomination process with each of the entries.

Here are some examples of countries of the first type. This is evident from the list of former TWHS:

Argentina (3 nominated out of 5), Congo DR (2/2), Costa Rica (7/8), Croatia (2/2), Cyprus (3/4), Ethiopia (and Eritrea) (4/4), Latvia (4/5), Luxemburg (2/2), Norway (5/8), Pakistan (4/4), Ukraine (4/4), Uruguay (2/2).

Particularly for the early years all info re dates and T Lists needs to be read/understood with care for the various situations which could occur - in fact Pakistan has NEVER retracted a site from its T List.
Its first T List was submitted dated Aug 82 with 15 sites on it of which 6 - Shalimar (1980), Lahore fort (1980), Moenjodaro (1980), Thatta (1979), Taxila (1980), Takht-i-Bahi (1980) had already been inscribed!
At the same time it was in various stages of pursuing another 4 sites which were not shown on its T List. In fact 2 of these (Kirthar and Lal Sonhara) were rejected at the Dec 82 Paris WHC. The 1981 Bureau in effect "deferred" the Hunza rock carvings and the 1981 WHS papers mentioned that Kirthat, Indus Dolphins and Lal Sonhara had been received too late and would be considered in 1982 - what exactly happened re the Dolphins I don't know). Since these sites WERE nominated unsuccessfully and do not now appear on Pakistan's T List they appear on our list as having been "Former Tentative Sites".

In fact the other 9 sites on the original 1982 T List 8 are STILL there (Tomb of Hazrat Multan, Badshahi Lahore, Wazir Khan Lahore, Tombs of Jahangir etc, Hiran Minar, Rani Kot, Shah Jahan Thatta, Chaukhandi) - though on our site they have all had their "T List Submission" dates changed to a later T List proposal - 1993. For "accuracy" it might have been better to leave dates as per the original T List submission but UNESCO practice seems to be to show dates as at the latest submission so if earlier T List sites are included on later T Lists then the full picture is "lost"
Across the period since 1982 Pakistan has, however, added numerous other sites to its T List (Which now stands at 26!) whilst at the same time ONLY inscribing 1 site from the original T List Rohtas in 1997 and trying with Hiran Minar in 2010/11, failing TWICE on the basis of "lateness" and "incompletenes" whilst at the same time failing to score the "open goal" offered to it by UNESCO for the Badshahi Mosque if it would only nominate it!

So Pakistan actually demonstrates exactly the second of the approaches you have identified - for ever adding new T List sites without clearing off the old ones either by successful nomination or by removal from its T List!

I think some countries don't understand the UNIQUE value of a site, being very self-centered on what they think their national heritage is in terms of worldly status. E.g. Philippines recently cleaned up their list because pretty much every colonial town was thought to be WHS-worthy. But then again a lot of countries had a massive T-list like Germany and they were eventually reduced before even being nominated.

I think it is also "convenient" to some countries to have a big T LIst since it gives its states/provinces, ethnic minorities and towns at least the "feeling" that they are moving towards full inscription. Some governments can do this in the secure knowledge that, for many of the T List entries, they are unlikely to have to come clean and tell the "owners" that a nomination isn't going to succeed since those sites are unlikely ever to be able to put together a meaningful nomination without significant assistance. So - the provinces are "happy" that the value of "their" site has been recognised. In many cases the "people" are not really aware of the difference between T List "inscription" and "real" inscription - we have identified many examples on this forum of locations on T Lists which even market themselves as "UNESCO WHS" or quite "responsible" organisations (Nat Geog included) who get such things wrong. India provides great examples of Provincial politicians "blaming" Central Government, UNESCO etc etc for the fact that their province's particular site isn't progressing - it is NEVER their own fault!

Are there any stats available for what the chances are a 20/10 year old tentative list is actually going to be inscribed? I'm thinking that some are so outdated they will never be considered but they do sometimes re-appear.

We could take a sample with the 2020 nominations:
Amami-Oshima Island - 2016
Ancient Quanzhou - 2016
Arslantepe Mound - 2014
Chankillo Solar Observatory and ceremonial center - 2013
Classical Karst - 1994 (renominated in 2015)
Colchic Rainforests and Wetlands - 2007
Colonies of Benevolence - 2011 NL, 2015 Belgium
Cultural Rock Arts in Himã Najrãn - 2015
Darmstadt Artists' Colony Mathildenhöhe - 2015
Deer Stone Monuments - 2014
Fortress of Spinalonga - 2014
Frontiers of the Roman Empire - Danube Limes - 2009/2015/2015/2002 (differs per country)
Getbol, Korean Tidal Flat - 2010
Great Spas of Europe - 2014 mostly, except for Belgium 2008
Historical and Archaeological Site of La Isabela - 1990/2001 (long history in various forms)
Holqa Sof Umar: Natural and Cultural Heritage - 2012
Kaeng Krachan Forest - 2011
Le phare de Cordouan - 2002
Nieuwe Hollandse Waterlinie (extension) - 1995
Padova Urbs Picta - 2006
Ramappa Temple - 2014
Site of the Retiro and the Prado in Madrid - 1995 in an earlier incarnation
Sítio Roberto Burle Marx - 2015
The Work of the Engineer Eladio Dieste - 2010
Trans-Iranian Railway - 2017

It seems that:
- 5 to 10 years is a normal expectation
- 2 sites are really fast tracked, Quanzhou and the Trans-Iranian Railway
- 15 years or longer however is not that uncommon (6/25)

Published 2021 Nominations (missing limited number of natural nominations)

Cameroon / Chad / Niger / Nigeria - Lake Chad Cultural Landscape (2018)
China - Badain Jaran Desert (2019)
Chile - Settlement and Artificial Mummification of the Chinchorro Culture in the Arica and Parinacota Region (1998)
Cote d'Ivoire - North Ivorian Sudanese Style Mosques (2006)
Egypt - The Coptic Monasteries of Wadi al-Natrun (2003)
France - Nice, capital of tourism in the Riviera (2017)
Germany - ShUM Sites of Speyer, Worms and Mainz (2015)
Germany / Netherlands - Frontiers of the Roman Empire – The Lower German Limes (2018)
India - Dholavira: A Harappan City (2014)
Iran - Cultural Landscape of Hawraman/Uramanat (2007)
Italy - The Porticoes of Bologna (2006)
Japan - Jomon Prehistoric Sites in Northern Japan (2009)
Jordan - As-Salt The Place of Tolerance and Urban Hospitality (2015)
Latvia - Grobina archaeological ensemble (2017)
Mexico - Franciscan Ensemble of the Monastery of Our Lady of the Assumption of Tlaxcala [Extension] (2018)
Russia - Petroglyphs of the Lake Onega and the White Sea (2018)
Slovenia - Ljubljana: The Timeless, Human Capital Designed by Jože Plečnik (2018)
Spain - Ribeira Sacra (1996)
UK - The Slate Landscape of Northwest Wales (2012)

< than 3 years = 8 nominations
< than 10 years = 4 Nominations
< than 20 years = 5 Nominations
> than 20 years = 2 Nominations

That would be really remarkable! But I guess there are a few renominations among them. Grobina dates already from 1996 for example.

Originally from 1996.


Extended from a 1987 entry.


Originally from 1996.

This changes the statistics:

< than 3 years = 7 nominations
< than 10 years = 4 Nominations
< than 20 years = 3 Nominations
> than 20 years = 5 Nominations

Site of the Retiro and the Prado in Madrid was updated 27/01/2015, the previous documentation was probable horrible and would have never been acceptable. Ribeira Sacra is still a joke, is this actually examined this year?

Chinchorro, simiarly, but I remember reading Chile is doing some pre-analysis with ICOMOS and perhaps that doesn't require new descriptions.

I don't think this site was ever on the Tentative list before. There was one for the Palacio Real in Madrid, but that was a sepreate entity I think.

Oh no I take it back, it was indeed propossed, it is on p18 here:
Espagne Salon du Prado, Musée des Sciences et Jardin Botanique
As found by jonathanfr a few years ago

As it was one of those from the depths of the early t-list I think you are right to assume the documentation would be dreadful/ non existant.