UTF-8 slugs for discussions are trivial, since lookup can be done by the ID part.
Tags does need to be converted to use the extensible slug system. I don't see any technical reason not to do so, except that we'd need to rename the
That all being said, flarum/framework#3429 is a bit concerning: we'll need to figure out why it's failing to query by a unicode slug. Perhaps we might need to add a URL decode step? Either way, if we convert it to our current slug system, that could be incorporated into the driver.
I think it would be good to offer UTF-8 slug drivers out of the box for users, discussions, and tags. It's a widely requested feature, and has limited overhead. If we make our system compatible with it, no reason not to support it.