Hey, welcome to Flarum!
I think it's moreso that there haven't been strong use cases in favor of this. It's definitely possible, and we've been working in a direction that would make that even easier, but there are several issues in the way:
- From a performance perspective, having very many tags scales worse than it should. We've done a lot of work to improve this, but there's still a fair bit left to go, especially when it comes to more efficiently loading paginated sets of tags from the frontend.
- The permissions UI, as well as the admin UI, assume that the number of tags will be relatively small, and become effectively unusable with very many tags.
Partial solutions to both of these that would unblock having massive numbers of secondary tags wouldn't be too challenging, but they would take some time to implement, and seeing as there hasn't been massive demand, we've been focusing our efforts on other initiatives.
That being said, there's also a hashtag extension: https://discuss.flarum.org/d/27476-hashtag, but it's more oriented around searching, and isn't too formalized (there's no robust DB index of all posts in each hashtag). Unless your use case requires permission controls in these hashtags, this extension (possibly with some modifications) is probably your best bet.