> realistically Nothing uses them

Depending on how one defines "nothing," they are honored by XMPP clients.

CoreDNS in Kubernetes also publishes SRV records, for any client in-cluster who wishes to look up the port number used by a named port on a v1.Service

XMPP, SIP, Minecraft, and Outlook are the four places I've seen SRV records supported, and obviously only two of those four are widely used with a variety of clients instead of just one specific client.