When a client tries to use a feature that's not in API Version 1, if the client passes apiVersion: "1" and apiStrict: true, the server replies with APIStrictError. In as many cases as possible, let's accompany the error with a message describing how to migrate to the Versioned API. E.g., when "mapreduce" returns APIStrictError, it should say, "Use aggregate instead", and link to the docs. Ideally, there is a page in the docs that helps users rewrite their mapreduce jobs as aggregations.
Similar for "distinct" (if we choose not to include it in Version 1), and other unversioned APIs with versioned alternatives.