They will probably keep the last version before the license change for a while, and probably add minor community-sourced patches.
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"
I think Debian would keep it around, probably, but it'll be punted out of the main repos and in to the ones designated for non-free software. Impact on the end user will likely be minimal. They'll still be able to apt-get install it as before.
See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=916107