It’ll work because the MTA supports catch all. While it doesn’t support VERP, it can be configured to forward all unspecified emails to a specific email, so the bounce replies won’t be lost.
Also, if we separate envelope-from and reply-to, envelope-from doesn’t need to contain a VERP address. It can be simple address like discourse@yyy.com. So when it bounces back it returns to discourse@yyy.com which won’t fail SPF.
The reply-to can have a different email which supports VERP like discourse-yyy+sdfkj33@gmail.com so if delivered successfully when the user replies it comes to the gmail server which supports VERP and it wont’ fail DMARC since the from is still yyy.com.
I agree but we don’t make 'em. Because they have catch-all addressing, they won’t support VERP. Unfortunately that the situation, I’m hoping that through a little tweak discourse will be able to support a more diverse ecosystem. A lot of smaller sites who’re using gmail to handle replies would be in this situation.