It has, e.g. in these threads:
- Folder tree view?
General Discussion
Jun '05 - Why this program doesn't have 'Folder tree view'? - Folder tree panel
General Discussion
Jun '12 - Hi Would it be possible to add folder tree panel? Changing directories would be much more comfortable that with the current button. - Feature Suggestion: Folder tree
General Discussion
Apr '11 - Hi! That'd be lovely to optionally have a folder tree on the left to save some time on Ctrl-D action. Thanks. - Edit TAG only F2 key; Internal music player; tree view for folders
Support
Mar '17 - ...tag it. and when you have a lot of tracks it's slow to play externally. Mp3tag has a tree view for folders ? it's much faster and intuitive to change the working directory by this metod. It will be so cool...
and more