Hi,
in general I appreciate that the Arduino-IDE offers support for so many boards.
Anyway I'm using just a couple of boardtypes but not all 2^8 of them.
I'm using Arduino-IDE 1.8.12.
So is there any possability to remove list-entries of this very long list?
Can this list be re-arranged that some boards are sorted to a sub-menu "other-board"
Can I create a "favorite-list"?
That could be the custom platform I mentioned (though you'll need to create a separate platform for each archtiecture, so if you're using multiple architectures then it probably won't do what you want.
What might be closer to what you seek is the "recently used boards" section of the Arduino IDE's Tools > Board menu, which is available in the beta build of the Arduino IDE:
There is even an enhanced version contributed by a community member:
but you'll need to build the IDE from source to try that one out.