It's not even that alone, it's app developers both dropping support for the old OS in new versions of their app, and then actively disabling old versions of their app.
This was what ultimately did in my last iPhone. I kept it well past when iOS upgrades ended, and over time apps stopped working. At first it was stuff that's pretty easy to let go of. My mobile provider's app was the first to go, but I could still do everything I wanted through their website. Eventually it starts to be stuff that's sort of the whole reason you own a smartphone instead of a dumb phone.
This was what ultimately did in my last iPhone. I kept it well past when iOS upgrades ended, and over time apps stopped working. At first it was stuff that's pretty easy to let go of. My mobile provider's app was the first to go, but I could still do everything I wanted through their website. Eventually it starts to be stuff that's sort of the whole reason you own a smartphone instead of a dumb phone.