After launching the initial test of its option to hide tweet replies to Canadian users back in June, Twitter has now announced that the experiment is being broadened to users in Japan and the US, a significant expansion of its test pool.
Starting today, this feature test will be available in Japan and the US.
— Twitter Support (@TwitterSupport) September 19, 2019
Now, you can hide replies to your Tweet, so you have more control over your conversations. Want to see all the replies? View hidden replies in the dropdown menu or through a new icon in the original Tweet. https://t.co/bnOmSzCP2f
As you can see in the below example from early testing, shared by reverse engineering expert Jane Manchun Wong, the process enables you to hide any reply from your tweet responses, giving you more control over how your conversations are presented.

And as per the last frame, users are still able to see any hidden replies, they'll just have to tap into the options to do so.
Here's a look at the current process in action for Canadian users:

The test has shown positive signs among Canadian users - as explained by Twitter:
"Based on our research and surveys we conducted, we saw a lot of positive trends during our initial test in Canada, including:
- 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.
- People were concerned hiding someone’s reply could be misunderstood and potentially lead to confusion or frustration. As a result, now if you tap to hide a Tweet, we’ll check in with you to see if you also want to also block that account."
(Note: You can see the new block prompts in the example above).
It's an interesting experiment, and as noted in the results thus far, it does appear to be having the desired effect, prompting users to reconsider how they respond and engage on the platform.
If it works to make Twitter a more civil, welcoming place, or even if it just helps some users feel more comfortable about their interactions, it's likely worth it, while the capacity to be able to view what's been hidden should minimize potential misuse.
Users in Japan and the US will be able to use the hide replies option in the latest version of the app.