I've been trying different things, including a new download, to get beyond the "File-Exit" problem illustrated in Post #18...no joy. 1.8.19 still works, however. All of this seems very odd to me.
Me too.
In most cases like this, I would recommend that you probably also post up on the Arduino forum as well.
https://forum.arduino.cc/c/software/ide-2-x/93/l/latest
You might include additional information as well, like what Linux and version are you running? AppImage or Zip format? Over there they will probably have you try to run the IDE from a terminal window or the like, maybe turning on some options and then posting the debug output.
Note: it is not just your tool menu, it looks like all of the menus are screwed up. That is your show: File, Edit, view, window, help menus.
But normally we have:

File, Edit, Sketch, Tools, Hep
If it were me, I would also probably hit it with large hammer. For example as Paul mentioned, delete your current 2.1 install, clean out your Temp folder, at least anything with the name including arduino, and the *.clangd files.
I would also clear out some of their cached information, like I need to do when I change a boards.txt or platform.txt file.
(issue:
https://github.com/arduino/arduino-ide/issues/1030)
In the case of Linux it says the directory. ~/.config/arduino-ide/
And then I would try to reinstall.
But before that I would check the Arduino forum and see if someone posted about this and if not post...
Edit: looks like you have posted:
https://forum.arduino.cc/t/2-10-lack-of-file-menu/1126014