LineageOS is an alternative operating system that is based on Android. At the moment it is the only option to get an Android-7-based #dic:rom on your Fairphone 2. An Android-8.1 -based release is in the making. This was mainly made possible by @chrmhoffmann, who did most of the porting, @snevas, who set up the OTA build server that facilitated updating to this point, and these great people.
Now the LineageOS port for Fairphone 2 is becoming officially supported by the LineageOS team.
PS.: Browse the tag #lineageos for more info on LineageOS.
PPS.: See the statistics chart by clicking the downwards arrow below (statistics taken from here):