Shut down of macOS takes forever
Solution 1:
The comment from @DavidSupportsMonica to try safe mode brought me on the right track. The problem did not occur with safe mode. As suggested here, I created a new user account with which the problem also did not occur.
Fortunately I remembered having registered jack to be started at boot, which turned out to be the culprit: jack simply won't stop. I guess at some point the shut down process reaches a timeout and just turns the machine off.
I've created an issue with brew-services.