What is the confusion? People on Windows 98 are able to send emails too.

People on windows 98 doesn't expect modern games to work for them. Many people on strange linux setups do expect games working on some linux distro to work for them as well and complains when it doesn't. Those people are the reason linux versions gets so many more support requests.

That's just about managing expectations, and it falls on you to be clear in your marketing. If you said "we support all versions of Windows" then you could expect someone to misinterpret that, the same way they would misinterpret "we support all versions of Linux." So really to make it clear, you might consider only saying "we support Ubuntu Focal" or something like that. Most products I see now make it pretty obvious that they only test on some combination of Windows 7/8/10 and above, so just do the same for whatever Linux distribution.