What application would you suggest for chatting / sending messages through TOR? With encryption? Is KDE's Kopete OK?
Tor Messenger. Kopete isn't OK because no one has checked if its designed to protect privacy.
I mean I have found a post in which PPL suggested other, better apps, but still in development. The thing is, it was a 1-year-old post. So I wonder what's the situation now? Is Tor Messenger still the bst Jabber client for whonix?
Tor Messenger is the best XMPP desktop client that's designed from the ground up around protecting anonymity.
The very best (non-XMPP chat client) option is something like Ricochet
BTW do you know how to check if I connected with the jabber server with SSL certificate in Tor Messenger?
The important thing is the E2E encryption layer between you and your contact. For all purposes you should assume the server is monitored and the alias is known by third parties.
I know how to use OTR. Just click the locker icon at the beginning of each conversation
But using SSL could potentially improve the security of my anonymity, especially if my jabber provider claims no to store any logs and is compatible with SSL
Not likely. You will probably be relying on CA SSL verification which is no good at all.