This is a valid concern. But I think we should reject this law on the basis that it should be a recommendation rather than a mandate, without rejecting its premise entirely, because it actually has some merit.
Filtering on client-side is a good idea and lets parents parent without affecting anyone else, provided the filters work completely offline. And we should make sure governments and parents know this so they don't try to push any more Internet-wide censorship laws.
And instead of mandating it, it should work like movie ratings: OSes that implement parental control features get a "PG-capable" label, then make it illegal for minors to use a non-PG-capable OS. This should not affect adults, and parents can choose to not use it because it's a feature you have to manually turn on.
