TL; DR: In this particular case, I don't see much backward compatibility, so yes, let's add them.
I've been thinking about the concept of adding methods (mixin or virtual) to ABCs in the context of the "views on sequences" conversation recently on this list.
In that context, we (or I, anyway :-) ) decided that it's potentially very disruptive to add a method (as opposed to a new protocol, via a dunder) -- because only the dunder "namespace" is reserved. It that case, there was a concrete example: a `.view` method added as a mixin to the Sequence ABC would be an obvious way to support the use case at hand, but it would then conflict with any Sequence-like (either registered with the ABC, or simply duck typed) that had a `.view` method with a different meaning -- and numpy arrays DO have a .view method with a different meaning -- so a lot of code could break. We could come up with a more obscure name, but then it would be less intuitive, and any name *could* conflict with something somewhere.
What all that means is that's a very big deal to add a new non-dunder name to an existing ABC, and we will probably rarely, or never do it.
BUT: This *may* be a different case -- the Set ABC (and set duck-typing) is probably far less used than for Sequences. ANd while the proposed methods are not part of the Set ABC at this point, they ARE part of the build in set object. And historically at least, people informally duck typed as often, or more often, than they subclassed from or registered with ABCs anyway.
All that is to say that there are no set-like objects in the standard library that have these names with a different meaning, and probably VERY few, of any third party set-like classes that have these names with different meanings as well.
So it would not likely break much at all if they were added. (and I'm still confused why they aren't there in the firs place, the PEP doesn't seem very clear about it)
And even if they are not added to the ABC, they could still be added to the other set-like objects in the standard library -- are there any other than the dict views?
-CHB