Yes. However, it’s not super difficult to get a signed image to meet the requirements. To my knowledge they aren’t actively trying to prevent the installation of other operating systems. The bigger issue is the software supporting their unique hardware.

I understand there’s quite a few missing drivers on the latest Macs. It’s possible to run Linux but I don’t think it’s especially user friendly at the moment. Apple does a lot of custom stuff and not all of their hardware has open source drivers available.