I can perfectly see this implemented without breaking backwards compatibility, similar to how we have usermodes for "connected via SSL" and channel modes for "requires SSL".
It doesn't require breaking backwards compatibility because, as far as outdated clients are concerned, they just "can't enter a channel because they lack a required usermode" or "can't send a message to some user because they lack a required usermode".
We only need the spec to define "some way" to do it so clients can announce their support and servers know what to do with the supporting clients.
Then it's up to IRC daemons to provide some modes for it.
It doesn't require breaking backwards compatibility because, as far as outdated clients are concerned, they just "can't enter a channel because they lack a required usermode" or "can't send a message to some user because they lack a required usermode".
We only need the spec to define "some way" to do it so clients can announce their support and servers know what to do with the supporting clients.
Then it's up to IRC daemons to provide some modes for it.