In this particular case those people may be right. That version of Safari will not load that site, and if you are not already starting to see other sites complain it is certain to happen with increasing frequency.
Quick fix: use another browser. https://bayarearidersforum.com/forums/ loads just fine using Brave for example.
To download Brave, start here https://brave.com and click the Download Brave button. I confirm Brave will work on your Mac's Safari version, and that site will load just fine.
Better, long term fix is to upgrade macOS for that Mac Pro, but you will first need to install a Metal-compatible graphics card. For faster and better instructions than I can provide, consider posting a brand new question in the Mac Pro Community. But for now, use Brave instead of Safari for that site, and others that may begin to complain.
Good luck with your mc.