They had a very long and comprehensive manual. The manual on each page inter-linked easily to switch between the relevant page for different major versions with a dropdown version selector (3.x, 4.x, 5.0, 5.1, 5.5..etc).. and if a page had moved or didn't exist it always accurately redirect to the correct page as you did that switch.
And almost every single engineering change that ever mattered to me made it into the changelog and also had relevant docs. I could largely rely on it and didn't need "git log" like I mostly need today to figure out what changed in a version.
Partly this was process, every closed bug/change went to the docs team to process.. but the team was also fantastic and converting that into relevant docs and writing great docs.
A shame if that has been lost, they did have a stack of layoffs recently in MySQL.. apparently the developer team is also heavily down from where it was. I am sure this writing is a little biased but interesting reading never the less: https://mariadb.org/reading-the-room-what-europes-mysql-comm...