That is truly detestable. I have been on that forum, and it is in general a pretty useless one IMO. But to try to thwart it, use a "user-agent switcher." It will fool the website into thinking you are on Windows 10, with a recent version of your browser. You want the one by "Ray." It is an add-on to Firefox and other browsers. It often works. I'm actually running software that should not work on Windows 7, like Vienna Instrument libraries. But it doesn't always work. In which case you can just forget about that largely useless, and now ridiculously restrictive, forum.
Addendum: If you really need that forum, you may have to bite the bullet and install Windows 10. Now that it's EOL, once you have it updated all the way, and configured to remove the bloat, Edge and other undesirable microsoft mischief, it can be somewhat tolerable.