To clarify: this applies to a Mac that has been locked.
That is, if the Mac is locked with Activation Lock, it wouldn't be possible to install another OS; the firmware itself will lock the user out of the computer entirely until the machine is unlocked. This dissuades thieves from stealing your MacBook as it will effectively be useless for anything other than parts, and most thieves aren't in the tiny-amounts-of-aluminium-relative-to-if-they-just-stole-cars recycling business.
I clarify because I don't want anybody thinking one is entirely unable to install another OS at all. That is possible, but of course you lose out on macOS features like Activation Lock.
That is, if the Mac is locked with Activation Lock, it wouldn't be possible to install another OS; the firmware itself will lock the user out of the computer entirely until the machine is unlocked. This dissuades thieves from stealing your MacBook as it will effectively be useless for anything other than parts, and most thieves aren't in the tiny-amounts-of-aluminium-relative-to-if-they-just-stole-cars recycling business.
I clarify because I don't want anybody thinking one is entirely unable to install another OS at all. That is possible, but of course you lose out on macOS features like Activation Lock.