You deload when you stall multiple times on a lift and the stall cannot be attributed to gains on another corresponding lift.
Example: Your military press is does not increase this week you are at 150x8 150x6 150x4. The following shoulder workout you press 150X7 150x5 150x4. Finally, you have another shoulder day and the reps stay the same or decline even further. If diet and rest are in order then it’s time for a deload.
Another example: Your press has been increasing every week. However, your lateral raises have been stuck at the same weight. You perform lateral raise after pressing. You do not need a deload in this case since the iso movement is not truly stalling, rather the compound is increasing and placing extra fatigue on the shoulders.
Deloading should be reserved for when you truly need it. Otherwise, you are just wasting time. The same goes with the over-training myth. Most people will never over-train to the point of serious cumulative fatigue. Usually the best treatment for suspected “over-training” is to decrease the volume and increase intensity.