If the new MongoDB licence is non-free, the chances that Debian will include that software in their repos are nil.
See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=916107
The next version of ubuntu will include mongdb version "3.6.9+really3.6.8+90~g8e540c0b6d" which you can see means "last commit before the license change"