It appears that X is working on the reply sorting feature, and new mentions of the functionality have been added within the back-end code of the app.
X aims to introduce options that would enable users to sort replies to posts in-stream, and thus surface the most personally relevant engagements of a post.

X has been working on it for some time now with this recent example showing how the functionality likely to work in live deployment.

In February, X News Daily also shared an example of how X's post reply sorting may enable users to filter comments by engagement.
That could make easier to find the most interesting, or more recent responses to any update in-stream.

I'm not sure if that's still in the pipeline as part of this update, but the idea would be through this ability for X's systems to offer personalized presentation of reply threads on posts for all uses, and that they may then increase in-stream activity.

Recently, replies have become more of an important emphasis for X because its Creator Ad Revenue Share program allows verified X creators who are accepted into this initiative to potentially receive a share of the ad revenue of promotions that may become displayed within the replies of their posts.

This has concentrated more on replies as a posting strategy, so it's reasonable for X to include additional ways of focusing in on this aspect and help creators understand better what might elicit a reply.

Reasoning why sorting by engagement makes sense in a broader sense is that this new display seems to focus more on how to customize reply presentation according to personal engagements.
Which is a slightly different approach, and maybe, in testing, X has found that this is the way to push relevance and interaction better, because it can rank post replies.

Or, conversely, it could also make echo chambers easier still, by favouring content from the profiles you interact with. And either way, of course, it all would depend on users actually using this ranking toggle, which they may not do.

But it's another experiment, which does now seem close to live testing.

It'll be interesting to see whether X users find value in the option.