RFC 7991, "The "xml2rfc" Version 3 Vocabulary", December 2016Source of RFC: IAB
Errata ID: 4904
Status: Held for Document Update
Reported By: Julian Reschke
Date Reported: 2017-01-15
Held for Document Update by: Robert Sparks
Date Held: 2018-02-09
Section 2.25.2 says:
Deprecated. If the goal is to provide a single URI for a reference, use the "target" attribute in <reference> instead.
This text doesn't make any sense, as the purpose for "alt" is something entirely different. The reason why it really was deprecated will have to be included here.