Tried and realised that Proton Mail supports WKD.

$ gpg --locate-key user@protonmail.com
gpg: key 4DE32C2A10A7EBC2: public key "user@protonmail.com <user@protonmail.com>" imported
gpg: Total number processed: 1
gpg:               imported: 1
pub   ed25519 2021-10-13 [SC]
      67731B189D0908618DF665144DE32C2A10A7EBC2
uid           [ unknown] user@protonmail.com <user@protonmail.com>
sub   cv25519 2021-10-13 [E]