XTLS has never been standardized, see https://datatracker.ietf.org/doc/html/draft-meyer-xmpp-e2e-encryption Gajim used this for file transfers, but there probably was no other client supporting this now obsolete feature. Today, using XEP-0396: Jingle Encrypted Transports - OMEMO would be the preferred solution.
Part of #11160 (closed)