1) Discussing endings is fine, it's the contents that are requested to not be posted. Why post the contents of the endings when you can already discuss it, especially under spoiler tags? In the same vein, because similar arguments are used, why would anyone need to stream the music room? The one requiring the most effort to avoid is streaming endings, and even that's trivial with at least OBS.
2) This will in all likelihood not be enforced (if so, minimally), and it shouldn't have to be. We as a community should be able to police ourselves with a decent level of respect.
3) Saying that you can already find the ending contents isn't a reason to further enable the spread of it. Trying to get rid of all instances everywhere has never been the point (and similarly for music room uploads). This argument without any further depth applies equally well to e.g. piracy, where because you can easily find it already there's no use in trying to stop it anywhere, which is obviously silly. In fact, I think having the endings being available to find elsewhere already is a good reason for trying to prevent its spread, as the fact that you can find it if you want should be enough for anyone interested, and nothing further is ever necessary.
