Is this just because they've tested it on modern machines?

I'm not sure, I had no idea Morrowind had received an update recently.

For what it's worth, at the time of writing, I can see that there are two versions available for download on GOG Galaxy: November 2024 and August 2015.

If 2024 version doesn't work, 2015 should definitely should work on older systems.

I don't know how long the 2015 version will be available for download, as some games had older installers versions removed (like Skyrim Special 1.6.659...).

Morrowind does not require Windows 10, but Galaxy may (I don't use it).

No it doesnt require windows 10. Gog just changed the system requirements because windows 7 is not supported anymore so they want to be covered in case people have any problems with the older OS