The link has basically been slug-less since the first version of this extension in 2016.
I created an issue to discuss the change FriendsOfFlarum/share-social13
I do think it would make sense to share the full canonical URL with Facebook and Twitter since they visually truncate long links anyway and don't have character limits on URLs. However increasing the character count could have an impact on the other social networks we currently support.
EDIT: I forgot to actually answer your last question. Right now Flarum officially supports linking to just the discussion ID and there's no plan to change it. It's used widely by extension developers to link their extension discussion threads, and we also occasionally shorten our own links on this forum by omitting the slugs. So there's no worries to have, the ability to link with either a slug or no slug is not going away anytime soon.
