The first four episodes of Star Wars: The Clone Wars on Disney+ largely centered on "The Bad Batch," an elite group of clone troopers that were nearly covered in prior seasons years ago, but the storyline was paused during the Disney takeover and the show was temporarily canceled. Since the return of new episodes of The Clone Wars, the Bad Batch story arc is very similar to the original story reels, with minor differences in characters and how the narrative wraps up.
The Bad Batch, until season 7 of was released, was one of several The Clone Wars stories not released but still considered canon. The Bad Batch is a group of clone troopers with "desirable" genetic mutations making them superior to most standard clone troopers in one way or another.
The original Bad Batch story reel was released online several years ago and can be found on YouTube, and the story, in most cases, is nearly identical, with several minor plot elements differing from The Clone Wars season 7's arc. One difference was Rex's emotional attachment to Echo and his concerns about Echo being alive taken far more seriously by his commanders than they were in the original story. The four-episode arc does well to characterize and humanize the clone troopers even more. The biggest difference in the story arc from the original is that Echo decides to join the Bad Batch at the end of episode 4.
In The Clone Wars story reel, viewers see the Bad Batch, Rex and Echo all standing in front of a crowd being awarded and commended for their victory. The version we ended up getting on Disney+ makes far more sense, though, with the Bad Batch opting out of an award ceremony because it "isn't their thing" and inviting Echo to join their squad, since he has become a misfit as well after his capture and experimentation on Skako Minor. Rex also gives Echo his blessing to join the Bad Batch, giving The Clone Wars season 7's story arc a happy ending and closure for all characters involved.
Some additional story reveals from other scrapped episodes have revealed that Echo was always intended to join the Bad Batch, and would have fought alongside them in other episodes, such as an arc taking place on the Wookiee home planet of Kashyyyk, but with the shortened season 7 episode count, those The Clone Wars episodes were likely scrapped, leaving Echo as a loose end to be explained at the close of episode 4 instead.