They should sign an ethereum address to reduce ambiguity
(Any crypto asset address is fine, even PGP is good enough for this but PGP had 25 years to make that user friendly and common but failed, and cryptocurrencies made signing software more prevalent and uniform wayyyyy faster)
However, there's still some very good programs with good UX making use of PGP (for example delta.chat), and to this day no cryptomoney scam wallet has ever been as useful as PGP has over the years.
More generally, there’s broad consensus in the cryptographic community that PGP’s intended uses and design are fundamentally flawed/mismatched against modern actual uses.
Don’t get me wrong! Cryptocurrency is filled with shysters and I don’t use any of them. But we should probably be encouraging users to stop treating PGP over email as if it does anything and instead encourage them to switch to E2EE systems (since that’s what the majority actually want.)