Why is this being filed now when LQT is on course to be undeployed, which will automatically get rid of the new messages feature?

Because we got message:

Hello everybody. Because LiquidThreads have been undeployed from Hungarian Wikipedia,

https://hu.wikipedia.org/wiki/Wikip%C3%A9dia:Kocsmafal_(m%C5%B1szaki)#Az_ideiglenes_fi%C3%B3kok_hamarosan_bevezet%C3%A9sre_ker%C3%BClnek

@sgrabarczuk was wrong. LiquidThreads hasn't in fact been undeployed from Hungarian Wikipedia. This can be checked by observing that it's still present at https://hu.wikipedia.org/wiki/Speci%C3%A1lis:N%C3%A9vjegy

Oh, I see that my message caused confusion about a feature I wasn't aware of. My apologies! The engineers are saying that they have converted all LQT pages, and they only need to set LQT to "frozen" (disabled), which is the last step. They will most likely do it next week. That was enough for me to believe that nothing is blocking the deployment of temp accounts scheduled for Nov 12.

@sgrabarczuk is correct - apologies for the confusion. Work on undeploying liquid threads will continue after temporary accounts deployment, e.g. T89426: Define LiquidThreads logs config and i18n messages in WikimediaMessages ahead of uninstalling LiquidThreads

Noting for posterity that this actually does appear to have been done. The "new messages" top-bar link and Special:NewMessages have both disappeared from enwiktionary, even though LQT is still enabled in a frozen state.

Odd. I have no idea why. The LiquidThreads repo has had no human commits in months.

Looks like that might be because WMF config appears to set $wgLiquidThreadsEnableNewMessages to false when LiquidThreads is 'frozen' for a wiki.
At the time of this task being filed, it seems like LQT wasn't yet frozen on huwiki -- it seems like that happened on November 3.