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.