Yes, biological science has changed in the last 150 years - contraception and safe abortive have both become available. This massively affected, for example, the very one-child policy you cite.
Likewise, the information revolution post dates both the examples you mention, also affecting the ability of the state to coerce. Ceauscescu’s Romania, in case you haven’t noticed, no longer exists.
I think you’ve confused child benefit and the child allowance for universal credit. It’s universal credit that currently has a limit on extra child allowances. I say currently because a bill to lift the limit is on its second reading.
Or, to provide a tongue-in-cheek essay question: “Does the introduction of an act to lift the ‘two child limit’ for UC in England and Wales suggest that the state in the 21st Century is not, in fact, able to control who has children and how many? Discuss.”