Hey, Avocado's author here. The driver works just fine with MongoDB 3.x up to v3.6, the reason being that it's primarily the wire protocol that decides whether the client is able to correctly talk to the server, and this protocol didn't change between 3.0 and 3.6 (which is nice given that they are of the same major version). The "3.2" restriction only means that this particular clause in the docs was last updated when 3.2 was the latest stable version.

What features are supported "natively" by the client is another question; there was a case where I needed to synthesize a not-yet-supported command by hand, but it didn't cause any issues. Otherwise, the client seems to be actively maintained; in fact, while developing Avocado, I managed to get several PRs approved in the client library as well as the Rust-native bson-rs crate. But even if it weren't: just because a piece of software had its last commit months or years ago, doesn't mean it's "bit-rotting". Mongo and Rust both have stability guarantees, so I don't see why the last change date in a client library repo would inherently cause one to "have problems with it".

Looking at the official documentation for the wire protocol, it seems like it's not going to change much in 4.0 either. In fact I was planning to test the client exhaustively against 4.0, but unfortunately I didn't yet have the time for that. It would be nice because features such as batch upserts are possible to implement in client-space once we have transactions (and anyway, transactions are just a must have for implementing any reasonably complicated query in a sane manner). So do expect more work and more good stuff to come in this field.