Well, it looks like the rumours were somewhat true, ANBERNIC are indeed working on a slide up handheld, and have officially teased it on their official YouTube channel.
One of ANBERNIC's owners has been talking about dual screen handhelds and slide up displays for sometime now, hinting to their community that it's coming soon, and it is indeed coming soon!
The RG Slide looks like an older slide up phone from the late 2010's, featuring nothing but a display on the front, and a slide up mechanism that will likely reveal the controller.
Only the display and the back has been revealed so far in this teaser, but it gives us a first look at the design, the size and its shoulder buttons, making it look very similar to the PSP Go.
Now, I am guessing here as no official information has been released, and the model could be tiny or 8ft tall, but to me, from reflections and seeing how it looks in her hands this is another pocketable device, likely with a 3.5" - 4" display.
It could feature the same 3.4" display from recent products such as the RG34XXSP and RG34XX, or a 4" display used in the recent RG406H.

It depends on where ANBERNIC is going to price this, and if they start it as a premium device with a premium price like the RG CUBE, which could follow the same path and get a more affordable version in the future, such as the RG CUBE XX.
But this is just me dumping my thoughts, but its nonetheless cool to see these Chinese manufacturers start to innovate and jump into the sliding and clamshell handheld scene.