I don't think was ever more than a set of SARPS.
Airlines can apply it, or not. Almost all do, but none are required to.
Agreed.
But I've never seen any evidence of airlines ignoring its provisions re flight numbers, which is what we're talking about here, probably because chaos would rapidly ensue.
Imagine looking on an airport departure screen for your flight number and finding that there's a BA123 departing this morning to XXX and another BA123 this afternoon to YYY.