HMD Global locks the bootloader tighter than Fort Knox. Unlike Google Pixels (fastboot oem unlock), Nokia requires an that only HMD can provide. For two years (2017-2019), HMD allowed unlocking via a developer portal. They shut it down in 2020.
Until then, the community is keeping the torch alive. The Snapdragon 660 powered devices (Nokia 7.2, 6.2) will likely see custom Android 15 builds via Project Treble GSIs. custom rom nokia