It seems that Nokia Mobile is on a roll to clear the backlog of pending source code releases. The open source webpage is updated again today, and we can spot the newly listed Nokia 5.1 Plus. They have uploaded the Kernel source code for both major releases: Oreo and Pie.
Nokia Mobile is legally required to upload the parts of open source software that they use in their devices. So far they have uploaded the source code of 5 new devices in the first ten days of January. So, hopefully, all of Nokia smartphones will have the parts of their source code available for download by the end of January 2018.
You can download the source code of all Nokia smartphones from the Open Source page on Nokia.com.