As long as only those wanting to make use of the new feature
need a newer SDK, I have absolutely no problem with protocol
extensions which require changes to existing SDKs.
Certainly an old client should choke if they were provided
an LDAP URL with a subordinate scope (through user input
or from the server)... but an old client should also choke
in face of an LDAP URL with a critical subordinate control.
And, I note, both likely might require SDK enhancements
(most SDKs handle LDAP URLs returned by the server directly).
To me, the implication is that servers should only return
such URLs when the clients specifically request that they
be returned.