Option "Register Teams as the chat app for Office (requires restarting Office applications)" not kept
This option in TEAMS Parameters is always flagged when TEAMS is started. So no way to set LYNC as default IM instead of TEAMS. You always have to start TEAMS, unflag this parameters before starting OUTLOOK if you need IM integration with Skype for Business.
Robin K. commented
By raising an incident to Microsoft I got feedback from the product team that this is by design if you are running your MS Teams environment in the "Teams only" mode instead of using one of coexisting modes. So in this case it is "by design".
Currently we do have a non Microsoft IM tool as default.
I can not believe that this is the truth. I just asked ones again. Unbelievable this forced overtaking of a role in a system.