That's true for devices which come pre-loaded with an OEM version of a Pro edition of Windows, allowing downgrades to the equivalent editions in older Windows versions that have not yet reached their end of support dates (so right now Windows 8 or newer), and also true with different guidelines for licenses covered under the Volume Licensing or Software Assurance programs. However Microsoft doesn't always make it easy to legally obtain the media for versions of Windows older than the one before the current version, which is another prerequisite for legally using older versions unless you already have the necessary media.

Downgrade rights would not allow use of Windows 10 for anyone who buys a Windows 11 upgrade license from Windows 8.1 or older, a Windows 11 full packaged product retail license (as one must do to legally run Windows on a Mac), or a devices which comes OEM-preloaded with a Windows Home edition.

Other situations are less clear to me.

No way you can get XP to accept a W11 license key though

It's been so long since i did any of that. I believe it did work though, or if not then you could get MS to issue you a real one.

They might not do it now.

I do remember being surpised how far back Windows keys would work.