On a another thread, @CWallace reposted on how to download official older versions. To determine the previous version you used last on macOS check the second highest revison numbered folder either in:
~/Library/Application Support/Plex Media Server/Updates
or
~/Library/Application Support/Plex Media Server/Crash Reports
In Finder’s Go menu, select Go To Folder… then enter one of the above in the dialog box that pops up.
If you’re using FreeBSD, you can also download older versions from pkg and ports. Details available from FreshPorts:
