I have seen them before but I didn't know they were called.
The HP G5 DL380 has something like that but I thought it was non standard.
HP 408788-001 ProLiant DL380 G5 PCI-X Riser Card
I 'll have to check to see if the slot dimensions and pins are the same size and the controller.
That's a PCI-X riser, PCI-X is 64 bit PCI (conventional pci), not pci express.
PCI-E bifurcation requires chipset and bios support as far as I know, it doesn't work on any board.
There are also adapter boards which have a pci-e to pci bridge chip, so you can convert one pci-e x1 slot into multiple pci slots.
ex. pci-e x1 to 2 x pci :
https://www.amazon.com/Taidda-Stable-Extend-Adapter-WIN2000/dp/B083F8QW6B/Considering a single pci-e slot has 250 MB/s (pci-e 1.0) , 500 MB/s (pci-e 2.0) or ~970 MB/s (pci-e 3.0) of bandwidth, a pci-e -> pci bridge chip will have plenty of bandwidth for 2+ slots (each 32bit slot has 133 MB/s max. bandwidth)
There's also pci-e to multiple pci-e boards, ex pci-e x1 to 4 x pci-e x1 :
https://www.amazon.com/Cablecc-Express-Switch-Multiplier-Splitter/dp/B073QNW3L9/These are sold under various brands, they're all clones of same product. they were popular during the mining craze, as it allowed running multiple video cards with cheap motherboards.