Investigate NODE-4397 - can't use string as ObjectId

NODE-4397 Description
Just upgraded from 3.7 to 4.0.1 (and then 4.7.0, but the same issue remains).  I'm using TypeScript, and am using strings instead of ObjectIds for my _id fields. After the upgrade, I get the error:
  Type 'string' is not assignable to type 'ObjectId'
when I try to do something like:
  await db.collection('mycollection').insertOne({_id: 'abcdefg', foo: 'bar'});

are string IDs not supported anymore?  Or is this just an omission of the type definitions?