If you’re looking for an Intel CPU it could be that they’ve moved on. It could actually be the same for AMD boards though I would expect some to still be around.
My board with legacy PCI was for socket AM4 and the chipset is the first generation chipset for AMD’s Ryzen. They now have moved on to a second generation chipset which is slightly different, and it’s possible manufacturers have moved on.
I think ultimately you just have to hunt a bit and maybe settle for an older motherboard, which could be totally fine, or try one of those bridges/adapters… or sell the card and get a new one…