I wanted to provide one last update to this thread. I use my ISY to control my Insteon devices. I don’t use any of the other features of the ISY (I have moved everything else to HA). I do use Insteon events (DON, DOF etc.) to create timers etc. and I do have some scenes setup in the ISY and control them from Insteon.
I took the plunge and removed the entries in configuration.yaml that were there to establish the connection to the ISY itself (38 Insteon devices, so it it was going to required manual changes it would have been a lot of work). Upon restart, everything worked with no need to any manual work and the warning went away. So it was as seamless as “shbatm” indicated it would be for my configuration.