The government legislating APIs is an uncomfortable precedent given the culture wars that are raging right now. There seems little reason to expect this will stop here.
I work in aviation, a highly regulated field. And that's a good thing. It does take some work to regulate well; there has been a migration in aviation to more prescriptive regulation about how things need to be, to less prescriptive like what the ultimate performance needs to be. But yeah, the aviation regulations aren't that you have to implement something a specific way, but that you have to be able to show that your aircraft has no more than a certain probability of catastrophic failure (where the probability varies base on certain things like the size and type of aircraft).
For this age verification law, all that is required is that there is an API provided for this purpose, and there is a way for the owner of the machine to set up user accounts with age information indicated, and that the APIs need to provide several rough age ranges, not specific birthdays.
It might also include some additional text like "we have decided to collaborate with systemd to integrate this proprietary binary blob, to maximize the reach and eliminating any pains in the setup process caused by the vibrant ecosystem of package managers, while at the same time avoiding disrupting the development process of the Linux kernel".
We shouldn't object to a reasonable law just becasue it might, theoretically, pave the way to an unreasonable law.
In fact, this is put in place as an alternative to the kind of law being enacted elsewhere, right now, which is much worse; the ones requiring ID based verification for accessing many online services. This one provides an alternative solution, which is far more privacy preserving, and leaves all of the actual power in the hands of the owner of the computer.
Are these illegal operating systems?
Either you or someone else mentioned this talking point the other day, I asked for even a single example of an OS maker being sued over this successfully, and I got nothing.
