It's meaningless to state "regardless of SQL engine platform" as MINUS is only valid for Oracle. The equivalent for SQL Server is EXCEPT, and I don't know if MySQL has an equivalent or if is supports EXISTS(). In any case, even if you find a syntax they all support, none of them will optimize or process the SQL the same way, and performance will vary.
By the way, the LEFT JOIN syntax would be better classified as a "conventional JOIN", as it is supported by the ISO SQL standard:
SELECT D.deptno, D.dname
FROM dept D
LEFT JOIN emp E ON D.deptno = E.deptno
WHERE E.deptno IS NULL
ORDER BY D.deptno;That is somewhat more likely to work on all 3 platforms, and actually uses JOIN syntax. They still won't process the same way, although their optimizers may construct the same or very similar plan to any of the other forms you listed.