I think you can actually see it in the graph.
The Condor Cluster with its 500 Teraflops would have been in the Top 500 supercomputers from 2009 till ~2014.
The PS3 operating system is a BSD, and you can see a thin yellow line in that exact time frame.
There’s no reason to believe smaller supercomputers would have significantly different OS’s.
At some point you enter the realm of mainframes and servers.
Mainframes almost all run Linux now, the last Unix’s are close to EOL.
Servers have about a 75% Linux market share, with the rest mostly running Windows and some BSD.