As noted at the end of the article, I suspect the impact for many OS's is going to be that they add a line in the fine print somewhere saying not for use in California.

They could just comply by prompting the age of the account user in a text file. File that the user is free to edit.

Already the case for MidnightBSD.

You're assuming they don't want this just as much as the government. Still feel fine about self-installed Linux, but every OS and device we don't have control over, even ones powered by Linux, will be very happy to include it, assuming it's not too difficult to add.