Is out of date diclofenac harmful?

Very much depends on the medication. Some can lose their effectiveness whilst some may even become harmful: many on the other hand will survive beyond the use by date for some time so the safe option is play safe and dispose of it.

In the specific case of a named substance, such as diclofenac, the information about how long after the published expiry date it could be used will not be published outside of the pharmaceutical companies producing it.