Proposal to remove Sprout fields from the V5 transaction format.

[nuttycom]

This modification is suggested as a possible simplification as a result of this discussion in #446: https://github.com/zcash/zips/pull/446/files#r585949598

[daira]

This proposal would prevent direct migration in a single transaction from Sprout to Orchard. I think that's fine, but we should have more consultation with the community about whether it is ok from a privacy point of view to migrate funds immediately to Orchard, rather than via a ZIP-308-like mechanism. (That's technically independent from this proposal, but interacts with it.)

Given a positive response from such consultation, I support this proposal because it sends a clear social signal that Sprout is "on its way out".

[dconnolly]

I support this very much. Tying Sprout deprecation to V4 transactions alone, and when they are no longer supported, is very elegant from a maintainer's perspective to be able to just turn off anywhere V4 is supported and that turns off new Sprout processing. Also I think it will be an nice communication tool to communicate the how of Sprout deprecation to a community ('v4 transactions are going away, they are the only way Sprout funds can get out of the pool' or something like that).

[teor2345]

I think this is a great way to communicate another step in the sprout deprecation process: transfers out of the sprout pool are supported, but we're making concrete deprecation plans.

I also really like the ability to disable transaction v4 as a way of disabling sprout.

[daira]

Need to add the ZIP 222 reference and check the encCiphertext offset. Other suggestions are non-blocking.

[daira]

ACK. I'll generate the HTML and merge this.