Sometimes this is for legitimate technical reasons due to older models not being able to support certain new features, e.g. The reality is that Apple has almost always deliberately killed off support for older models after a certain amount of time. See - How to get macOS Sierra - Apple Support Thou must join the heathen masses in purgatory running El Capitan for ever more. Apple have doth decreed that the Mac Pro 2009 is BS - Before Sierra and therefore unclean 😝.
