The problem is that it’s hard for us to _really_ measure intelligence.
One idea: intelligence approximates someone’s fitness — their ability to survive in the future.
There’s no way to know this though, so better to just let all live and let nature take care of the selection.
Doing it for <30 (hell even <50) is completely useless.
Even amongst population groups whose average IQs are in the 70s or 80s, a vanishingly small percentage of people exist below that range.
From what I've read on IQ literature anything below a 60 is an estimate as the subject lacks the required intellect to comprehend and complete an IQ test. On the ethical aspect I don't think mandatory sterilisation should be done for any reason including intellectual disability.