Many motherboards do allow splitting the GPU lanes into two it's just not many consumers see the need for an NVMe drive +GPU+ 2nd high speed device + 3rd high speed device that can't be served off the chipset instead.

For the prosumer crowd if you do something like the z790 you can have x4, x8, x8, and x8 to the chipset to be split as needed (x16 to be split if 3.0.

If I understand correctly, you're concerned about using older peripherals with a newer motherboard/CPU? If so, the PCIe spec requires backwards compatibility so it should still work.

A gen3 x4 card in a gen5 x1 slot gets... gen3 x1 speed. Even though gen5 x1 has the same bandwidth as gen3 x4.

So, is there such a thing as an adapter that would plug into a fast narrow slot and provide a wide slower slot with the same overall transfer rate? Or, provide multiple slower (but still just as wide) slots from one faster slot?

A PCIe switch does just that. It connects at its native speed then each port is backwards compatible effectively making wider effective slots if you connect older things downstream.

It can be hard to sort between expensive/high end enterprise stuff from low end single lane stuff for crypto mining when trying to find something useful though. NVMe m.2 storage is the best chance of finding cheep consumer options but even then you have to sort through the endless bifurcation cards to find the ones that are actual switches though.