Tags used in private staff categories don’t appear in the counts on the public tags page to non-staff users, which makes sense. So a tag only used in a staff category doesn’t appear on that page at all.
However, a tag added to an unlisted topic does show up on the tags page for anonymous or non-staff members, but the topic doesn’t appear on the page for that particular tag.
Oh good - a topic about bears:
Oh pook - no, there isn’t.
It doesn’t matter whether the topic was tagged before or after it was unlisted; the behaviour is the same. (This happens even if the topic and tag were created by a TL4 member, who has the link to that topic in their profile; the member can see the tag, but not access the topic from that page.)
It seems to me it would make more sense to treat tags on unlisted topics in the same way as tags on private topics, and only show them to moderators/admins.