It seems odd that their spec uses /Users and /Groups rather than something like .well-known/scim/Users. It seems assured that it will conflict with existing routes on many applications.
I’d be happy to help out with that if you’ve got some budget to spare.
I can imagine that if it were well-written and widely-used (or required by several enterprise customers) that it might become an offical plugin.
I don’t imagine that it would ever become part of core–even Oauth2 is a plugin.