Description:
We should change the documentation for 'failIndexKeyTooLong' server parameter and tell the user that there will be no size limit for index key in MongoDB 4.2. More accurately, 'failIndexKeyTooLong' will still be effective in MongoDB 4.2 (with FCV 4.0) but completely ignored in MongoDB 4.2 (with FCV 4.2).
Engineering Ticket Description:
It would be confusing to have this server parameter when there is no length limit for the index key. Deprecate this server parameter so that it is ignored when the FCV is set to 4.2.