Posted by Early testing of this feature in Canada showed positive results:
People mostly hide replies that they think are irrelevant, abusive or unintelligible. Those who used the tool thought it was a helpful way to control what they saw, similar to when keywords are muted.
We saw that people were more likely to reconsider their interactions when their tweet was hidden: 27% of people who had their tweets hidden said they would reconsider how they interact with others in the future.
https://blog.twitter.com/en_us/topics/product/2019/controlexpansion.html