Changing the fingerprint size makes v5 incompatible with existing OpenPGP cards
I've been made aware that changing the fingerprint length is problematic for the OpenPGP smart card protocol. If we want v5 keys to be compatible with existing hardware - and I think we do as not to hamper adoption - we should give guidance on how to interact with that hardware.
I'd propose to truncate SHA2-256 to 20 octets for the benefit of communicating with such cards.
See page 23 of https://gnupg.org/ftp/specs/OpenPGP-smart-card-application-3.4.1.pdf.