The whole MediaWiki Language Extension Bundle (https://www.mediawiki.org/wiki/MediaWiki_Language_Extension_Bundle) should be updated, I guess.
But I went with updating the Translate extension anyway.

Please confirm if the tvar syntax now works.

I do not know whether the server has already been updated but it seems the new syntax is still not known by MediaWiki. See https://lingualibre.org/index.php?diff=683554 for example. Maybe @Eihel knows a bit more. At https://www.mediawiki.org/wiki/Help:Extension:Translate/Page_translation_administration#Segmentation, it is written that the new syntax is supported since Translate version 2021.04 but on https://lingualibre.org/wiki/Special:Version, I see that Translate version 2020-04-20 is installed.

I have updated the Translate extension only. It is now on the head of the REL1_35 ref (latest commit: Oct 28 2021), but it didn't show as updated on Special:Version.

So :shrug: until I investigate it further. As i said previously, it might have required to update the whole MLEB, which I have not done.

Talking of MLEB, it seems it has received an update this month. I'll definitely have a look at how I can update it safely on Lingua Libre, this week-end. And hopefully you'll get access to these much-wanted tvar tags ;)

I am not sure this is related to this ticket but a Lingua Libre user is not able to translate the interface into the Pa'O language because this language is not available in the list of languages. See https://lingualibre.org/wiki/User_talk:%E5%92%BD%E9%A0%AD%E3%81%B9%E3%81%95#Add_Pa.27O_language_and_Tai_Ahom_language

I do not know if it is still time push this update with the MediaWiki update planned in a few months.