I think they already allow modifications? Apple lets you disable System Integrity Protection [0], after which point you can make any modifications to the system. Only the T2 supports Secure Boot and it can also be disabled [1].
"Activation Lock" is an iOS thing. On Macs you can set a firmware password [2], but I don't see how that would conflict with GPLv3.
If a judge and jury find that the T2 chip, macOS, and bash4 were all integrated components of the software package “macOS” for the purposes of GPLv3 copyleft provisions, then Apple would be forced to share the source code of all software running on the T2 chip as well as all software it bundles with macOS.
While we can armchair legal this in either direction on how likely that judgement would be, they appear instead to have decided to never permit that risk to occur at all. I approve, too: if you don’t want to share every bit of source code in your product, don’t use GPLv3 in your product. It’s explicitly meant to be annoying like that, and any plausible effort to have your cake and eat it too risks being overturned by a court of law.
While we can armchair legal this in either direction on how likely that judgement would be, they appear instead to have decided to never permit that risk to occur at all. I approve, too: if you don’t want to share every bit of source code in your product, don’t use GPLv3 in your product. It’s explicitly meant to be annoying like that, and any plausible effort to have your cake and eat it too risks being overturned by a court of law.