It’s still an operating system. Not implementing something is saying “this OS is not to be used in a country / state with age verification laws” Basically baring anyone in california or wherever implements these laws from using the OS in a legal way. I suspect most of these OS’s (even ones that are not “under US jurisdiction”) are going to eventually do something like when you install it asks where you are located and if its in a location where age verification is required it installs the age verification system.

It’s not like OpenBSD is exempt from the law. If they aren’t implementing some version of it, they are just hoping no one enforces anything.