> I recall these used to be in a Mediawiki
> page somewhere, but now I see they
> are only available on GitHub?

Wikibase is in kind of a weird place being presented as both an extension to MediaWiki and a product in and of itself. MediaWiki has a more organised setup (but only somewhat, I've helped fill out the changes on wiki once or twice): https://mediawiki.org/wiki/Release_notes

They look better but are ultimately similar in content (but bigger since they cover all of MediaWiki core).

> Interesting how all of them even in Master
> write "This is not a release yet!", seems they
> were never finalized and cleaned.

They were finalized only in the release branches/tags, by the looks of it, as part of the release process. Perhaps "merge changelogs back to master" could be added to it.

> How reliable are they to contain all changes made?

I gather the focus is on end-user visible changes, e.g. new visible features or API changes, especially those which might break existing code.

In particular https://doc.wikimedia.org/Wikibase/master/php/adr_0017.html outlines the release notes template which includes the following difference from MediaWiki release notes:

Don't include "New developer features" or changes to "development-only external libraries"
One main difference between MediaWiki and Wikibase is that Wikibase is an extension to MediaWiki and our external developer community is not as big. To reduce the burden on the team we think it's a good idea to not include these.