Hey,
I just downloaded the newest Unity SDK 2.6.0 package from the ML Package Manager. It seems like it's provided as a preview package (specifically 2.6.0-pre.R15, see screenshot).
Not sure if this is intended. Checking out the package on the npmregistry it has the correct release version 2.6.0.
No big deal in general, I'm just curious to find out if this was planned and if the contents are indeed from the same revision.