The slide is narrower than an actual RMR (which is an actual brand model, not a generic term for red dot sights), but you can put an RMR on a 43X if you don't mind it sticking out on the sides. I personally don't care for the look or the fact that it makes a gun I bought specifically for it's narrowness wider. The Trijicon RMRcc or the Holosun "k" series are made to mount on more narrow slides.
This is the sight Glock set the gun up for https://www.shieldpsd.com/ That said the folks on the Glock forum are questioning the choice they are saying it is on the cheap side.
I have been putting red dots on subcompacts for a few years now. It all started with the G43, then the P365, then the P365XL and finally the G48. I never went with a Tricjicon RMR on any of those guns (even though it is my go to red dot for all compact & larger sized guns). I just never liked the look or the ergos of the sides of the red dot being wider than the slide.
For my G43, P365, P365XL and my G48, I put on the Shield PSD RMSC red dot. I had issues with the first versions because the polymer lens was very prone to scratches. Then they came out with the glass lens version and that is much better.
However, I am now in the process of replacing all my Shield RMSc red dots with the Holosun 507K. I think it is the best micro red dot for a pistol on the market. Just my 2 cents. BONUS! It fits the same pattern slide cuts as the RMSc
But what about the RMRCC? I think it is HIDEOUS! And with the proprietary mounting pattern, I would have to buy a new gun just for the red dot! NO. I also cannot believe they did not go with a side battery tray. swing and a miss.