Hi,
Looking at the doc, it seems if I use only one NVMe SSD, I will get only 2 lanes and have 2 lanes unused for the empty second NVMe slot ?
The 5B non + version had 4 lanes and a single NVMe SSD slot, but it would get the 4 lanes.
So this is a downgrade in terms of single NVMe speeds… correct ?
Technically it is a downgrade but in what situations would you even notice/care? The sequential speeds look nicely in marketing material but in normal use they aren’t the most important. Unless you routinely move several hundred GBs.
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 
If your running the latest and greatest, but… gives excellent speed and price that means eMMC in a way doesn’t make sense. Likely why the eMMC is solderbare as you get x2 that you can stripe or mirror.
You get x2 budget level new NVME in either configuration, ‘X2 speed’ or redundancy that match the hardware in use, that you can replace. For some that is a big plus for a wide range of uses.
Then also there is similar hardware schemes even for wireless…