Maybe (at least partial GEOB) support could be added to Mp3Tag with an option to read and display GEOB contents? (Or just an indication, that file actually contains GEOB data).
Currently mp3tag completely ignores them, except when removing tags when it removes GEOB as well and user has little or no clue whatsover that file even contained GEOB.
This is so.
And to expand the list of possibly present but completely ignored fields:
PCST
TDES
TGID
TKWD
WFED
can only be treated in the same way. WHich is also valid for all the priv()-fields
Maybe a new Mp3tag function ...
'"frame_exist(file_type|tag_type,frame_name)" ...
could help to detect such invisible data, ...
in order to prevent one or more unsupported frame/s from accidental removing.
DD.20150721.1155.CEST
Well, just a warning when removing tag`s would be nice for a start. (Ability to actually remove absolutely everything, may come handy to someone as well)
"Warning, file contains additional unsupported tag fields. Really continue?" 
Please not that it is not only possible to remove all fields but you can define positive
https://docs.mp3tag.de/actions/remove-fields
and negative lists.
https://docs.mp3tag.de/actions/remove-fields-except
Like this you should be able to keep also unsupported fields for treatment with other programs.