v2.2.1 does not start up in Debian Buster
;-) That would mean to support SRV records.
The same draft specifies that the current method, Enigmail v2.0.10 implements, by using https://domain//.well-known/openpgpkey/hu/encodedHash is the fallback variant, if the primary one: https://openpgpkey.domain//.well-known/openpgpkey/hu/encodedHash?l=user failed. Looks like, downloadWkdKey() needs to query the long one first and on error the short one.
The same draft specifies that the current method, Enigmail v2.0.10 implements, by using https://domain//.well-known/openpgpkey/hu/encodedHash is the fallback variant, if the primary one: https://openpgpkey.domain//.well-known/openpgpkey/hu/encodedHash?l=user failed. Looks like, downloadWkdKey() needs to query the long one first and on error the second one.
WKD implementation does not honor _openpgpkey._tcp.<domain> SRV records