I have a 5 year old android phone that I'm super happy with (still fast, OLED screen, cameras on par with current phones, 2 day battery life etc.). Apart from the fact that google has started to kill it. E.g they won't allow banking apps to be installed, because it's android 11. So One+ was one of the brands on the shortlist for my next phone. Guess they're off now.

I'm confused, I have a OnePlus 13 and it says I'm running 16.0.2.403

There isn't an update available at this time. So should I just not update if prompted?

They’re not blocking custom ROMs, just ones with older blobs. Right?