Sounds Like An Oxymoron ...

but there is a group working to make it happen. Smartphones cycle into end of life quickly. The hardware of your smartphone cannot get a new Android version. It isn't an option for you. Running the old leaves you vulnerable to security holes both in the OS itself and the applications which themselves often cannot be upgraded because the newer versions of the software you want will only run on the newest OSes.

@ Enter Lineage OS.

Check it out. It gives you the option of installing on some smartphones. Some day it could make higher end smartphones far more attractive.