if we look at the philosophy behind self-hosting - to be able to host it ourselves, aka not rely on an external party for hosting, that means that if an external party is down or otherwise unavailable, then that line becomes fairly apparent. If 3rd party auth is required, then it's not really self-hosting then is it, because if that third party is down and the software can't be used, then that philosophy is violated. if it's optional, then that's fine though.
For something to be considered self-hosted, If the bombs drop and civilization is over and me, and my home lab are the only survivors, can I still use that software? I'd probably have other concerns in that case, like how am I going to survive nuclear winter, but that's where the imaginary line is.