I don’t feel like my iPhone 12 mini is slow with 17.4? I guess it doesn’t make sense t argue about subjective experiences but I just wanted add a some anecdata.

Battery feels just fine (over a day, that is with Tailscale turned off [did get better some updates ago.])

That's strange. I'm on iOS 17 on my iPhone 11 Pro and it's just fine. Maybe the battery issues mentioned are due to normal degradation?

Do older versions of iOS still receive security patches?

not really