My private data is on each one to some extent. But only the 6s+ leaves the house and that's for two 30 minute walks around my neighborhood each day. It's the only one with a SIM, the rest are on my home WiFi.I'd not put my private data on it. If used to play music or a flashlight... keep it.
And to be real honest, only the 6s+ is actually used with any frequency.