Given they don't even support 5120x1440 on displayport on Mac OS, while bootcamp windows can on the same hardware, Apple just is not prioritizing external hardware that isn't in their store.
On a 2019 MacBook Pro 16" with the dedicated GPU, I've had success connecting a 5120x1440 Samsung CRG9 via DisplayPort. I had to use this adapter [0] and this cable [1] to do so, but it does work. It also works on a 2018 MacBook Pro 13", but due to the lack of a dedicated GPU (i.e., Intel graphics only), the output is a bit fuzzy as it's not a natively supported resolution. Still, it's good enough for productivity work, though not for video/photo editing or gaming.
In particular, my MacBook is the 2015 model, 15 inch.
My solution is switch res x with two outputs, 2K each, and picture by picture mode. However it has problems. Like you can’t zoom cast a window that overlaps the center.