I would like to have the patch notes in the changelog in the library. Searching for the patch notes in the forum or on Steam is a bit annoying.

Devs have nothing to do with this. Only the GOG team that works on the website can update the library changelogs for game x.

For this game you can find the changelog always in the main menu.

I am not interested in "Buchbinder Wanninger" or "Passierschein A38" games. It's not my job to do this. This task is either up to GOG or the devs. They should have an interest in ensuring that the products are properly presented.
GOG's support is not very responsive.
Also, I have read somewhere here on GOG's forums that devs are able to edit the pages for their games, not only Galaxy's backend. I know. But that was not what I asked for.

Yes they do. The developers are the ones that update the patch notes for their game.
I've literally talked to developers about this. Notice how 3d realms has never even posted here once, and surprise, they don't update their patch notes for their games either. They are an inattentive developer when it comes to gog, always have been.

Most devs are like this, but there should be a rule for developers to pay attention to their product if they are going to collect money during development. One man development teams are capable of doing it, they have no excuse.

No they don't. Devs are responsible to keep the changelog up-to-date but they are not able to integrate those updated changelogs into the library part of the gog website or in GOG Galaxy. That part can only be done by the people from the GOG team that are actively working on the website and/or Galaxy.