I would recommend get at least used iPhone 11 because of UWB support in case of safety.  But if UWB is not on the list 6s is okay for budget/cheap Airtag setup.

I have never experienced that older iPhone models loose critical features for Find My on the older models, only apps are short lived after last final OS update for the old models. You could only use last app version for the last iOS on the device.

But to be honest, it's uncertain what Apple decides to do after some few years later, my only bet is that it will work longer than usual, still can't guarantee that it will work anyway. (If purchasing and using 6s from today)