This will probably set off a huge debate, but I've heard a lot of people talk about it, and I think there's some merit to it.
Does a high megapixel camera really perform any worse in low light compared to a low MP camera? One big caveat I want to say is this though, I'm talking about at the same viewing size!
For instance, while a 1DX MkII undoubtedly goes to a higher ISO than a 5D4, since at any given viewing size the 1DX MkII photo is zoomed in further towards its native resolution than the 5D4 photo, the IDX MkII photo will show comparatively about the same level of noise, since you aren't zoomed in as close on the 5D4 photo to view it at the same size.
I remember somebody posting similarly sized photos from what I think was a 5D4 and a 5DS, and it was apparent that, due to the added size and detail you get from the added resolution on the 5DS, it pretty much negates any advantage the 5D4 might theoretically have in low light.
Essentially, even though each pixel is smaller and doesn't collect light as well on a high MP camera, you end up with what amounts to about the same amount of noise as a lower MP camera, for all practical purposes.