Some older devices are supported through some weird unofficial sites where you have to send in your IMEI, which is obviously shady...
Also they don't update their kernel sources in a timely manner at all. Which makes developing custom ROMs WAY harder..
So sadly, Nokia sucks if you wan't custom ROMs, I ditched my Nokia 8 for a Poco F1 for this reason.
The older models used Cyanogen and then moved over to Lineagos
Cheap and cheerful.