It's great that you brought up this topic.
AFAIK, legally any work is under copyright by default and can't be used without permission, we can not change that. Yet it would be useful to have a way to find works for which certain permissions were granted by the authors.
For works licensed under Creative Commons there is a metadata standard that popular search engines already understand (at least for images). But when I last tried to include metadata in a post, the Steemit parser ate all the extra attributes. It would be great to have this fixed. And even better would be to have an option in the submission form that will automatically add the necessary metadata — it will help to make more users aware of the possibility.
Common tags would also be useful for finding reusable content, but probably only as an ad-hoc solution until licensing metadata is fully supported for posting and searching. For CC-licensed works we can use one of cc0 (Public Domain), cc-by, cc-bysa, cc-bync, cc-byncsa, cc-byncnd (by = Attribution, sa = ShareAlike, nc = NonCommercial, nd = NoDerivatives) and maybe also a catchall #creative-commons.