It's used by importers, importing systems with a hard-coded Jabber field.

Ahh okay that makes sense, but given that Jabber.org are still going strong I don't know why this field is reserved for imports. Was it always that way? I recall the field used to exist on the edit form.

"From August to November of 2017 we ran an experiment with manual account creation. Based on the results of that experiment, we plan to implement automated account creation in early 2018." - Jabber.org FAQ page

Not something I use personally nowadays though I think I have a Jabber ID somewhere from back in the 90's and it still seems popular enough that Cisco have software for most O/S's.

https://www.cisco.com/c/en/us/products/unified-communications/jabber/index.html

If it's changed, it changed a long time ago.

v11 is handling social CPFs completely differently, as discussed in other issues.