I was convinced to advocate for deprecating XHTML-IM by Waqas Hussain, who found serious security vulnerabilities in the wild among a number of XMPP clients. Although in theory XHTML-IM might be "nice", as Sergei put it in this thread (and I was always quite fond of it myself), if in practice client developers can't or won't implement it safely then we shouldn't bring it back from the dead.
I understand that this has been the argument but I do not find it convincing. People will find ways to implement every XEP in an insecure fashion. Many vulnerabilities have been found in HTTP file sharing implementations but we do not deprecate HTTP. Security vulnerabilities hrve been found in implementations of https://xmpp.org/extensions/xep-0393.html but we don't deprecate that either. HTML does not have any more likelyhood of causing security issues than these others. All have beee abused in ways thrt caused issues and when we become aware of these it is important to document them of course.