GnuPG allows the user to sign a key with a non-revocable signature. I have done this when I’m signing one of my own keys—signing my personal key with my work key, for instance. Is there any other situation in which this kind of signature either should or should not be used? Are there any commonly-accepted semantics in a non-revocable signature?
RFC4880, 18.104.22.168. Revocable considers non-revocable keys a "commitment" to a signature that cannot be revoked. The certification's validity can only be invalidated by revoking the entire key used for signing.
I cannot think of a bunch of use cases apart from yours of non-revocable signatures to other keys you're owning yourself.
I could imagine use of this flag for special signatures, like in a situation where an employee designates a revocation key (like an OpenPGP key acting as corporate CA), which cannot be removed any more (so the corporate CA is definitely able to revoke the key, even if breaking up with the employee in conflict.