I voted harmless.
Where did this doubt start? Wasn't it one rabbit (or was it a cat, can't remember) that had some medical issue and licked too much salt, and that story went viral, got repeated and distorted so it's an "internet fact" now that salt licks are dangerous?
Mine have a block of stone salt, if they need it it they lick it (salt is essential for mammals), if not they ignore it. I couldn't tell if they licked it, since the amount they need is miniscule, but as long as there isjn't some (imho very rare) medical condition that makes an animal crave salt I would say it's harmless.