Parmandur
Book-Friend, he/him
Yeah, this sounds wrong.
The first UA was July 2015, the first Mystic. Then another version in February 2016, and a final version in March 2017.
It doesn't look like any of these versions passed muster, as Wizard's said in the last UA that feedback was too negative and they've given up on the Mystic.
Then in 2019, Sorceror gets a psionics subclass. The next month, November 2019, Wizard/Fighter/Rogue get subclasses too.
Now we have April 2020, we rerun of the fighter/rogue/sorcerer.
If you look at this timeline, it looks like this;
1. The team kept trying to find a way to create a Psionics subclass, and it was never able to cross 70%.
2. The subclass route has better feedback than the Mystic, and is why they are continuing to pursue that route for psionics instead of a different class.
3. Some subclasses are getting more editing, including the fighter/rogue/sorceror.
Now, is the reason the subclasses are getting more editing because the feedback is too negative, and they are trying to edit them to get them above 70%? Or is it instead that they dropped Wizard because it is the worst received, and tweaking the other three because they are close to where they are happy with them?
I suspect it is the last, that the rogue/fighter/sorcerer are close to actual being ready for print. The Mystic also went under several iterations, but each one was released almost a year apart from each other. Compared to that timeline, these subclsses are getting a much faster UA testing period, hinting that the team is pushing for a book soon that puts in psionincs. Considering the last version in April is more tweaks than complete revamps, it looks like they are getting close.
As @Parmandur says, people on forums don't represent all D&D players. If material is good, it gets printed. Plus, if Wizard's really wants to publish something, they may be more flexible about that 70% threshold. They don't even submit every new rule for review, as Satyrs and Leonin are being published for Theros and never saw a UA.
Now, I don't actually know if this truly means Dark Sun (though please, please Dark Sun). There haven't been any UA's yet for races or other Dark Sun-related material that I've noticed, though they possibly are just saving those (Theros' new races never got testing) as they don't want to give the game away.
I'd be amazed if we got these rules this year, even in Q4. Next year Q2 makes more sense to me, but I'm hoping I'm wrong and it is sooner.
The fact that Psionics hasn't come fully online yet isn't for lack of effort by WotC: they've iterated and reiterated. Which they'll do until they hit on what works for the audience.
Based on the feedback cycle for past years, what they are testing now is under consideration for the Fall book, whatever that might be. I'm feeling either Xanathars Guide 2: Electric Bugaloo it Planescape, but WotC have mastered the left field surprise.