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" 🙂
Zombies will despawn overtime unless they are holding items. They won't turn into drowned unless they are fully submerged underwater.