This is not downgrade, but just different product with it’s own features. I suggested radxa team that they may consider option to allow switching between 2x + 2x config and 4x in one of selected slots, but that did not happened so for (not only in radxa sbcs). For now if You need four lanes in m.2 slot just choose 5B instead of 5B+. You can still add bifurcation card and split that port if this is needed, but for sure it’s bigger and bulky solution compared to two slots in 5B+.
Like Łukasz said - for nvme it’s not that important unless You have good reason for that. If You plan to connect regular pcie card then double check what is it limit (some have 4x pcie configuration, but can’t reach such speed). Of course there are some cards that can get everything from such port like this monster 