https://jira.mongodb.org/browse/SERVER-74167 shows that when the `explain` field of `aggregate` is used, an API Strict Error is thrown. However, the `explain` field of the `aggregate` command is not listed here: https://www.mongodb.com/docs/manual/reference/command/aggregate/#stable-api while in the changelog it is listed as not guaranteed to be stable. Should update the list of unstable fields for `aggregate` to include `explain`.