[tomjen3]

I am sure you get Lineage OS, or something like it to run great on an old phone, just as I am sure you installed Debian yourself.

Mostly when we are talking about phones getting upgrades or not getting upgrades any more it is about updates from the manufacturer, so I don't see where you are going.

[throwaway81523]

I think that is not so easy but I haven't looked into it too much. Only a limited set of phones work with it and I think their functionality is also limited. I do have a Nokia N900 from around 2008, but despite its coolness it has never really been usable (too slow and too small).