I don't think such an extension exists at this time, but it's relatively simple to implement through Flarum's model slugger mechanism. Most of the challenge is to decide on / offer a choice for the suffixing logic, whether to re-use slugs when a discussion is deleted, whether to allow moderators to modify the slug. Also if the title of the discussion gets edited, should the slug change and if it does, additional logic will be required to redirect the old URLs to the new ones. That's a lot of additional complexity that the ID-based solution doesn't need.
There was this now-abandoned extension that I created a while back that allowed setting such a slug, but only manually, as the goal was to create pretty URLs for specific discussions https://discuss.flarum.org/d/25993-manual-discussion-slug