[ZIP 316]: Relax the restriction on Revision 1 unified addresses to permit transparent-receiver-only UAs.

[nuttycom]

This is a contentious issue that we've gone back and forth about. I'm creating this issue to register my objection to the current restriction in https://zips.z.cash/zip-0316#requirements-for-both-unified-addresses-and-unified-viewing-keys:

Consumers MUST reject Unified Addresses/Viewing Keys in which the same Typecode appears more than once, or that include both P2SH and P2PKH Transparent Addresses, or that contain only a Transparent Address.

I assert that this is the wrong choice, because one can always include a Metadata Item that is not a Must-Understand item, and any wallet will interpret that address in exactly the same fashion as a transparent-only address. I do not suggest that the quoted constraint above be modified to demand the presence of a Must-Understand item; instead, I think this restriction should be removed, because the fact that UAs cannot fully subsume other Zcash address types makes it more difficult to transition all users of the network to using UAs. This is the same reason that I objected to the manner in which ZIP 320 addresses were introduced.

[daira]

Yes I think we'd reached consensus on this and I'd withdrawn any objection to making this change.