4.0 is a thing.
Azure does a document store as well. It also claims 4.0 compatibility.
If it really implements 4.0, it's just about useful, as you do get aggregation pipelines.
Amazon's DocumentDB database service is described by the cloud corp as "MongoDB compatible", but MongoDB CTO Mark Porter has told The Register this is not entirely the case. MongoDB CTO Mark Porter MongoDB CTO Mark Porter at AWS Re:invent DocumentDB is for storing, querying and indexing JSON data and its full name on AWS is …
As someone who is trying to interface with CosmosDB, there is fucntionaility missing in the APIs.
MS actually state the rationale for using the Mongo API on CosmosDB is there for the benefit of using Mongo-connection-capable clients against CosmosDB.
As a use of several clients, I call BS. One client makes a call to the UserInfo function, which Cosmos does not support
Amazon may not even want to implement the complete feature set, just those relevant to their service & customers. SSPL is great if you don't want certain companies to use your software, but it is not a surprise if they come up with different workarounds and alternatives as a result. This surely creates confusion on compatibility and scope of features, especially if these are implemented according to the interests and needs of one specific company, like Amazon, rather than the broader user base.
We are running an open source (Apache 2.0) project called FerretDB which is set out to come up with a MongoDB compatible proxy, and there is a lot of traction on it.
https://github.com/FerretDB
Full Compatibility is unlikely to matter. I think MongoDB figured a great pattern about how to conveniently work with Document Stores from programming languages, and this approach, though not a standard is becoming akin to SQL for Relational Databases. And same as with SQL we will see some features being more important than others and full compatibility basically irrelevant.
I think AWS had a chance to make a greater impact in the ecosystem by opensourcing DocumentDB top level to let people run it with standalone PostgreSQL, yet as they did not do that it is not surprise solutions like FerretDB are popping up to provide MongoDB experience without pesky SSPL limitations.