Electrical panels generally should not have un-sealed openings for several reason.
One reason is that if there is a fire in a panel and there are holes, the fire may flash through the holes. These holes can usually be sealed with metal snap-in plugs.
When snapping in these metal plugs extra care should be taken, especially in an electrical panel or sub-panel. At times they will not snap directly in and pop out while snapping them in. If they hit a live bus bar or certain other parts of a panel, then arching may occur.
A second concern is that if they are in a hard to reach location or near live power that is not shielded, one could accidentally come in contact with the live power and get shocked. If the metal plug is being placed into a junction box, there is much less chance of coming into contact with an exposed energized part or wire.
Terminology – Note that if searching on line a round metal plug is called a “steel hole plug.”
Breaker knockout ( filler plate ) missing
Breakers or rectangle knockouts that have been removed from a panel leaves a gap where a person or child could stick something in. Should it be metal, shock or injury might occur; therefore it is wise to seal these with breaker blanks that just snap into the openings.
The part of an electrical panel where rectangle knock-outs are missing is usually called the “Dead Front” and for good reason. ( READ ABOUT ” DEAD FRONTS” )
Terminology – rectangle filler pieces are often called knock-outs, panel cover twist outs or breaker filler plates. On line they are generally called filler plates or breaker filler plates.
Rodents and insects
Remember; holes may allow rodents and insects to crawl into a panel. Should this happen the rodents may damage the wiring or short out some circuits. ( Read About RODENTS DANGERS IN ELECTRICAL PANELS )