After #19360 I did a deep dive into other missing functionality
Some are postgres only, others are also not supported across multiple DBs.
The following seems to have enough viable traction to support IMO

Summary

Add support for EXCEPT and EXCEPT ALL set operations to complement the existing UNION, UNION ALL, INTERSECT, and INTERSECT ALL methods.

Database Support

Proposed API

$query1 = $this->Users->find()->select(['email']);
$query2 = $this->Admins->find()->select(['email']);

// Users that are not admins
$query1->except($query2);

// With duplicates preserved
$query1->exceptAll($query2);

Notes

• EXCEPT has broad support across all major databases
• EXCEPT ALL is not supported by MySQL - could either throw an exception or document the limitation
• Implementation should follow the same pattern as existing union(), unionAll(), intersect(), intersectAll() methods