I have implemented unique discussion slugs in the upcoming new BuiltWithFlarum website, but in my implementation each slug must be set by a moderator, and defaults to the ID. It shouldn't be too difficult to do the same with auto-generation. This could be something for a sponsored extension.
There's actually an issue with Flarum's URLs. While the discussion slugger can be customized, the regex for the router still expects the slug to begin with a number, and have a dash before any letter. This creates an issue for extensibility as the only workaround is to delete and re-add the discussion route which could introduce conflicts.
There's also always the issue of redirecting old paths. It's not an issue with Flarum's default slugger, but if you use unique slugs and allow editing the slug, you'll likely need to store the history and redirect old slugs. That's probably the biggest part of the work in implementing it properly.