o Removed features (obsolete):
    - Relays no longer support the obsolete TAP circuit extension
      protocol. (For backward compatibility, however, relays still continue to
      include TAP keys in their descriptors.) Implements part
      of proposal 350.
    - Removed some vestigial code for selecting the TAP circuit extension
      protocol.

  o Minor features (forward-compatibility):
    - We now correctly parse microdescriptors and router descriptors
      that do not include TAP onion keys. (For backward compatibility,
      authorities continue to require these keys.) Implements part of proposal 350.