If you're looking for an edge-mount header, or you want to submit a gerber with funky looking slots for "TH pads" to the PCB fab...
Some while ago I ordered a dongle for a J-Link, for that 2x10-pin male IDC header on these things. As you can imagine, the dongle needed to present the matching female side.
Whoever designed it just used
a female header with insulation-displacing knife contacts... and left the second plastic piece off, and soldered those knives to a PCB. The space between the knives, for the PCB to go, is about 1.1 mm, according to some dimension schematics I found on google. The dongle's PCB was correspondingly thinner than normal.
The pads for a footprint would be shifted 0.025" one way on the top side and 0.025" the other way on the bottom, for a 0.050" pitch, matching the ribbon cable. I might give a 1.2 mm PCB a try but that might be tight and bending the contacts.
Attached picture is from a random aliex listing, shows the insulation-displacing knives in detail.