Linux.com - https://www.linux.com/news/5-raspberry-pi-operating-systems-arent-linux-0
Looking for a way to get the most out of your Raspberry Pi? Running a project that just needs something more? Odd as it may seem, Linux might be the problem, so why not consider a non-Linux operating system? Several have been released, or adapted, for use on the Raspberry Pi.
1. Plan 9
Released as an open-source operating system in 1992, Plan 9 has a small footprint and is targeted at developers. Its lightweight presence makes it ideal for the Raspberry Pi.