I’d assume you can add any expansion card with drivers for Apple Silicon macos. Now that this config exists I guess some companies will make the necessary drivers for their products.
I think CXL devices in general will become more and more common given that other players on the hardware side are now supporting CXL 1.1 / 2.0 protocols (Sapphire Rapids / 4th Gen EPYC[2]) and it's a way to scale memory past the limitations of a single machine and the number of memory slots / DIMM slots.
For now it's PCIe 5.0, but given that PCIe 6.0[2] will come around with PAM4 signalling, FEC and FLIT mode on CXL 3.0 spec it may be not be as bad or slow with this future configuration.
Given that macOS has memory compression, it already has two "classes" of memory pages in a storage-hierarchy relationship, with active promotion/eviction. Just mount the CXL "external memory" to a different part of the address space, and default to writing compressed/cold pages there.
I'm guessing you can't use those PCI slots to add more ram or an external graphics card (maybe I'm wrong?!)
If not, what are they for?