They are currently on the Fairphone 6, and at least in Germany the official online store still sells a wide selection of parts for the Fairphones 3-6, and the display and camera for the Fairphone 2.

Sure, you don't get meaningful hardware upgrades (apparently there were some small ones), and Fairphone are far from the only ones selling spare parts for their phones. But in terms of keeping old phones alive with authentic parts and easy to execute disassembly steps, they are pretty good