I don’t think it’s possible at the moment and it makes permissions a lot more complicated.
Do tags allow you to see a topic (similar to category settings), or do tags restrict visibility of a topic? If a user is allowed to see topics tagged with “tag1” but not “tag2”, and a topic is tagged with both, what should happen?
If a topic is tagged with multiple tags, each restricted to different groups, how can we maintain a clear overview of who is allowed to view the topic? Will users notice if others may not be able to see the topic, or will they quote from it without realizing the visibility restrictions? The lock icon next to the category gives a hint for categories.
Nonetheless there is a feature topic
Restrict topic access by topic tag - user group