address-spec.txt defines .onion as only v2
This may have come up elsewhere, but looking through the issues here on gitlab for "Tor Specification" I did not see it.
The current version of address-spec.txt (which is used as the reference in RFC 7686) contains a few lines detailing the .onion https://gitlab.torproject.org/tpo/core/torspec/-/blob/ea4250ee823ddf31f29f479143ac293f99e25503/address-spec.txt#L54-55
The digest is the first eighty bits of a SHA1 hash of the identity key for
a hidden service, encoded in base32.
Strictly speaking this is not always the case, but is the case only for V2-onion addresses. Given the upcoming deprecation of V2 onions the Tor specification may require some overhaul (not only this example I found).
And also the reference to rend-spec.txt
on
https://gitlab.torproject.org/tpo/core/torspec/-/blob/ea4250ee823ddf31f29f479143ac293f99e25503/address-spec.txt#L58 appears to be outdated as the specification no longer have a rend-spec.txt but instead have a rend-spec-v2.txt and rend-spec-v3.txt.