Is there some old non-secure Jabber servers which still can be used with same obsolete clients in Windows 95 and Windows Mobile 5/6? Without all these SSL/TLS and such privacy paranoia?
Yes, you can find some of the servers with the old encryption at this link: IM Observatory.
I have registered accounts on jabber.sytes24.pl and on xmpp.kafuka.org (registration from XMPP client). I managed to correspond between them using the old j2me clients jimm 0.6.4m, LiteQD (a modification of Bombus from the Web archive), Jimm Multi. It also works with current versions of Psi+, Miranda, etc. The connection to these servers should also work on Windows 95, however, you should understand that these servers do not guarantee the security of correspondence.
Also for these purposes, you can use a service based on open-source analogue of the OSCAR protocol: KICQ