Well there's a mish mash. My Z590I VISION D does have bios options for it... IMHO there had to add some additional switch HW to have it working, basically fully route the RSV pins also.
For motherboards, it's entirely a software thing.
The host (i.e. chipset) has to support it, to grok what's being done w/ the different lanes. But given the preponderance of motherboards that have things like switchable slots (e.g., x16/x0 vs. x8/x8, which
is bifurcation), I feel safe concluding that most if not all modern chipsets support it.
The card itself (e.g., this asus one, or m.2 breakout boards) does require some extra hardware, specifically a clock redriver. But that's small and relatively cheap.
The only thing motherboards need to do is have their bios turn the feature on and configure it properly. So, as I said, all software.