Example:
I added status 16 to status 17 (workflowstatus table) in workflow 9.
I deleted status 17.
Rows with idStatusTo 17 remains in workflowstatus table.
The problem is that I no longer need status 16 in my workflow 9, however I can't hide it since status 16 seems to appear in the workflow for ProjeQtOr.

You can test with this request:
SELECT * FROM `workflowstatus` WHERE `idStatusFrom` NOT IN (SELECT id FROM `status`);

And this request:
SELECT * FROM `workflowstatus` WHERE `idStatusTo` NOT IN (SELECT id FROM `status`);