The usual IETF model is to define a core capability that has
the minimum
functionality to do something useful. Everyone claiming to
write code that
"supports" the specification MUST implement all of that
functionality.
Everyone.
Whether operators choose to use all the features is a
separate matter, but as
Mark notes, there is a problem if the initiating side uses it but the
receiving side does not.
The usual IETF model lacks a policy negotiation layer which is why it
has been impossible to perform any changes to deployed protocols.
DKIM is based on a policy negotiation layer that provides for the
missing mechanism for feature negotiation.