Just a quick followup: I did test the !notation on kbin.social. Their software is able to follow a !link to the destination. But the only test I did was for a community that was already linked to kbin.social. I have no idea what the user experience is like if they try to follow a !link to a community that has not been added to kbin -- or for a community that hasn't been previously searched for on the user's Lemmy instance.