While it would be useful in many cases, it could be harmful in some case: namely, when a translation variable is variable – a number regularly updated on the original page, a date, a user name changed every now and then, and so on. In this case, using an old version would be counterproductive, since the variable was introduced to avoid having to update the translations. (While such updates shouldn’t fuzzy translations, they may become fuzzy for unrelated reasons.)

Maybe instead of using the value based on the tp:transver, simply the latest available value should be used? This would solve the mediawiki.org problem without using older values than necessary.

However, that would break pages when numeric translation variables are particularly carelessly introduced and get reverted: I changed $1 back to $link1 because when replacing $1 with the link target, Translate also replaced $1 in US$100 million. Now, if Translate would use the latest available version of the tvar, it would break that amount of money again – and forever.

Would "if a tvar doesn't exist in the current version but does in the tp:transver use the tp:transver's version" solve all possible use cases? I think in the money case it wouldn't break anything that wasn't already broken.

There's also the third pattern to consider where a tvar is changed from an internal link to a URL or vice versa. But that's technically indistinguishable from the changing numbers case, so will just have to fail I guess.