Before 3.20, modifying the info for "Genre" was super easy. Then, 3.20 came around -- and, someone decided it was a good idea to bury the ability somewhere in Options . . . which I can never seem to find. Please, bring back the "old way" of changing "Genre" info. It was more efficient and better.
Don't know ...
Now you can add custom values with
Shift+Enter Add the contents of the current Tag Panel field to the list of custom list values for that field.
See also the documentation.
That was not possible before.
Also, it was not possible to add custom values to any other field - which is now.
The procedure is now the same for all tag panel fields.
Anyway - until it gets changed again, you would have to cope with the current method.
And that has been true for the last couple of years:
Mp3tag v3.19 was from 2023-01-27, two and a half year ago.
We are currently at v3.30d, 10 main versions and many subversions of Mp3tag later.
I would say this is highly subjectiv. 
Ctrl + O for Options, 1 mouse-click on the "Tag Panel" entry, 1 double mouse-click on GENRE and 1 mouse-click on the Manage-Button to manage custom list values is not really "buried somehwere" IMHO.
There is even a HOWTO Add custom user-defined Genres with pictures: