Windows 11 doesn't exist, there is only Windows 10. Windows 11 is a hardware requirement update sold as a new OS.
The licensing is the same, the system requirements are not.
And yes, Microsoft will not sell you an OS that leaves support in a few months. The lack of ability to purchase Windows 10 is proper, and I do not understand why anyone would be deploying Windows 10 fresh in February, when it dies in October. But, yes... you can use Windows 10 or 11 keys on each other all day long, because again, the licensing is the same.