mongodb inMemory engine available in the community edition?
As at MongoDB 3.4, no plans have been announced to make the In Memory Storage Engine available in the MongoDB Community Edition. I suspect this is likely to remain as an Enterprise-only feature for the foreseeable future given the more niche use cases for in-memory storage, however the definitive source will be the mongodb.com website and any future major release announcements.

FYI, you can subscribe to driver/server release announcements via the mongodb-announce group.