I have a lake near my village and zombies are getting trapped in it and not despawning.

I hear (and see in subtitles) "fire extinguishes" and "zombie groans".

Some of these zombies have been alive for over 12 hours (IRL time) so maybe a couple weeks in-game-time?

They don't move much but will track you.

[update] After messing around with villagers for a while, some of the zombies finally turned into Drowned. Is there some proximity component to the zombie->drowned transition? However I think the zombies should have despawned.

They appear to be stuck in 2 deep water and unable to move to 1 deep water.

[update] There are three left but one is holding rotten flesh so I don't think (s)he counts. The others are persistent buggers. Joke intended
On an only partially related note, I put my vote for the Drowned's vocalization subtitle to be "Drowned Gargled"