Currently, there are some tests (e.g., this one: "legacy documents might not have an _id field"? This is over 5 years old, from the initial implementation of change streams in 3.6. It does not seem to be relevant anymore, and there are a couple of places (e.g. updateLookup) where change streams rely on having an _id to work correctly.

As part of this ticket, we want to investigate whether there are any remaining cases where change streams may see a document without an _id, and how it behaves in those scenarios.