Is HomeKit now obligatory and why?

My iPhone 7 (iOS 15.8) is annoying me with this message.
I always leave iCloud services I don’t use turned off, isn’t that what the on/off switch is for?

I have never seen that message, and barely know what HomeKit is. I don’t have any options for HomeKit in the AppleID settings on my iPhone or my Mac. Have you used HomeKit at some time in the past?
My irritation is that I have to sign in to “GameCenter” every time I play a solitaire game on my phone (my default way of sitting in waiting rooms). If I don’t sign in, I am asked to do so, every time. If I do, I get a banner across the screen, ruining my time, telling me I am signed in. That is something I would like to nuke.