Thank you @JammyDodger for splitting this into a new topic.
I played around a little more…
…and opened the discourse instances on my mobile devices (Android and iOS phones). No menu items visible!
It seems that the feature is not there anymore which hid the menu items when making the desktop browser window smaller until the mobile view is active.
In my eyes this should be added again as all other components still work that way and activate the mobile view also in the desktop browser.
But for the “normal” user everything still works as it should.