alt Hacker NewsApple is on the record as being neutral at worst on the matter and at best weakly supportive. I think there was an article when the M1 came out where it was reported that the Asahi Linux folks met with some Apple developers where they were encouraged to explore the system and report bugs, but that Apple was not going to offer any support.
Apple has also done things such as adding a raw image mode to prevent macOS updates from breaking the boot process for third-party operating systems. Which is only useful for 3rd party operating system development.
Individual developers at apple may be weakly supportive (at best), but apple as a corporation has tended in the opposite direction, of locking down macOS and iOS more and more.
Sure, some developer may have added things like raw image mode, but if someone on high says "wait, people are buying macbooks and then not using the app store?" or as soon as someone's promo is tied to a security feature that breaks third-party OSes... well, don't be surprised when it vanishes. Running any OS but macOS is against ToS, and apple has already shown they are actively hostile to user freedom and choice (with the iOS app store debacle, the iMessage beeper mini mess, and so on). If you care about your freedom and ability to use Linux, you should not use anything Apple has any hand in ever.