If you are not familiar with the concept of backward incompatibilities, then you have probably not yet survived a major upgrade! To give you an idea of how important it is to be aware of this when reviewing the change log for MongoDB, this concept is also referred to as code breaks...as in things that can break your code.
The article at https://docs.mongodb.com/manual/release-notes/4.0-compatibility/ covers the full list of compatibility changes in MongoDB 4.0. For information on MongoDB 4.2 compatibility changes, go to https://docs.mongodb.com/master/release-notes/4.2-compatibility/#compatibility-changes-in-mongodb-4-2. For information on MongoDB 4.4 compatibility changes, go to https://docs.mongodb.com/master/release-notes/4.4/#changes-affecting-compatibility.
