I'm not sure there's ever been a Mac where you couldn't boot your own OS. Maybe the very, very early ones, but Linux for the later M68k and PPC Macs was definitely a thing.

I used to support a lab of triple-bootable Macs—macOS/Windows/Linux.

Current macOS is more locked down due to privacy and security threats primarily, not because Apple doesn’t want you to boot a non-Apple OS.

AFAIK this was always planned for the m1 macs

Personally, I will only consider a mac if there is the option to run Linux.

Mac.