There’s really no such thing as backwards compatible RAM. There have been some examples of platforms that support two types, like Skylake supporting DDR3L/DDR4, but that is based on the memory controller and there’s no signs that Intel or AMD is planning a repeat of it.
Alder Lake will support DDR4/5 on both mobile and desktop.
No laptop manufacturer would support both though, as they will have different sockets so you will always have 1/2 slots empty.