On the side panel, tags don't show the number of notes tagged with each tag (as it happens with notes in a notebook, as it's already been reported). Thus, you can't see at glance unused tags, in order to remove them
On the other hand, since the r-click context menu has dissapeared, I really don't know how to remove unused tags or tags mistakenly created
Acknowledging that you're trying to keep cross-platform compatibiliy, please fix the issue as good as possible