HMD Global may still be refusing to unlock bootloaders, but it's required to release each phone's source code by the GPL. The company has just published the kernel source for the Nokia 5.1, which was announced a little under two months ago.
For those unfamiliar with source code, it allows developers to build custom software like ROMs and custom recoveries more easily. However, because Nokia is refusing to allow its bootloaders to be unlocked, there's really not much that can be done. Nonetheless, if you're a dev and you'd like to check the source code out, you can do so through the source link below.