This is not the Plex server but the Plex client.
Either way: Windows 7 itself has been unsupported quite a while now. No patches for security issues are being produced for it anymore by Microsoft. Windows 7 support ended on January 14, 2020.
Continuing to run it is irresponsible when it is connected to the internet in any way. And it is connected to the internet in most cases when you use Plex on it.
Support of Plex software for an outdated operating system will inevitably cease. The plex player is the first to do so.
This is mentioned in the release notes Plex for Mac and Windows - #45 by mattseeley
