While many of us think GOG should really pressure DEVS to provide up-to-date changelog information, most of us realize they are probably between a rock and a hard place, so really it is the DEVs that are at fault, not GOG ... as far as we know.

There are places you can go to at GOG to find out the info you seek, though it may be incomplete or imperfect.

(1) The forum page for the game or series.

(2) A dedicated thread for missing changelog info.
https://www.gog.com/forum/general/the_what_did_just_update_thread

A short search of the forum would turn up many threads like yours from over the years, so you are not alone in your angst.

I think it's on the developers to update patch notes here, not GOG. Seems like it'd take a long of time to try to keep track of the hundreds (1000+?) of new titles here; each developer should be able to handle their own.

You're right they should be updated, but that's on the devs rather than GOG. And Steam is often even worse with the "update history" page ending up more of a news hub filled with advertising for newer games made by the same dev, the publisher "announces a sale", "livestream - Watch now!", etc) rather than actual game update changelogs. Example - see the unholy mess of how Mars Horizon's "update history / changelog" pages end up on Steam too...

That is correct. A long time ago I asked GOG about a missing changelog and I have been told that they only post what they get from the devs and if they don't get a changelog they don't post it.

Your example is a game released 3 days ago. Is there even a patch out for this game? I am talking about UBOAT for example, wich has recieved many patches since release. The newest is b128 hotfix 1. Yet, here the latest notes are from b126. This way behind the actual patch as far as notes are concerned is the case with nearly every game i own here since i joined GOG in 2016. Sometimes it took days or even several month to have the up to date changelog, wich was never the case on Steam.
GOG should and has to make sure that devs provide the changelog as soon as a patch is applied. If they don't, cut their profit or consider not releasing future games from them here. It is not up to us to copy paste such important information from Steam or any other site to the game related forums here.

Edit: I am well aware, that GOG is looked down upon by many actual devs and wannabe devs, but, if a dev can't be arsed to provide actual patch notes, wich are important in case your savegames are not usable with the new version of the game than those lazy bumps should be pressed to do so. Maybe GOG should include a contractual clause that devs must provide changelogs in a timely manner.

Just a note - Galaxy usually has a changelog/patch notes included with updates. It's always been there for for example Serious Sam 4 or Iratus: Lord of the Dead. So I feel like the devs put it there and just consider the matter solved.

So it seems it is mostly a matter of not posting them on the game specific forums. The community members usually do that but it would definitely be better to have them official too.

The website also has an entry for a changelog in the library - but they are still often missing.

Yes, changelogs are not updated in the games specific forum or in the library both on the website or galaxy client.