As far as I'm aware, they provide security updates to the current and the previous OS - but not sure if that applies to iOS or just macOS. I'm sure if they found something serious they would patch further back. But as most scams are aimed at fooling the operator rather than the phone, that's the big concern and Apple can't prevent you falling for something.
SE and 6s are very similar internally and a year more modern in architecture than the 6, which I'd avoid for that reason (SE is basically a 6s with some features removed and squeezed into a smaller form factor). I expect that they'll be supported for 4+ years yet, possibly more.
Apple is good at supporting older models and was selling SE until about 2 months ago. Personally, I loved my SE and found it perfectly serviceable( right up to a couple of months ago) but you might prefer the larger display of the 6s.
Apple usually provides security updates for "last year's" OS so that might give you a further year's usage.