Solved Movies & TV app

There is a 5 device limit for downloaded movies and the registered device can only be removed from Movies & TV app on the original registered device using Remove this device link and there is a limit of one device removal per month. What happens if the registered device breaks or is sold and is no longer accessible or its HDD or SSD is reformatted and Windows is reinstalled? Does reformatting the registered device also count as a new device to the Movies & TV app? If I reach the device limit I will no longer be able to download, only stream. Do unusable devices eventually get removed automatically after a few months of inactivity based on the last used date?

Not certain, but this sounds like a Microsoft account issue. In that case you should go to "Manage your account" in settings and remove any devices you don't have anymore or don't use. (A good thing to do anyway.)
If need be you could always create a new ms account for use with other devices.

There are limits to how many devices can use one MS account (10, I think) and also how many devices can use any particular app (I think that's usually 5, but it may be different for some paid for apps).

That only applies to the Microsoft Store apps but not for downloaded purchases or rentals of Movies and TV shows. These can only be removed from the PC that originally registered to download them.