[garren]

None of the above.

#1 is ruled out as he takes apart a number of plug receptacles, from “cheap” to “heavy duty” and “commercial grade”, none of them sport the bumps you’re describing. He also points out that not all plugs have holes (using an ikea timer as an example), and observes that most plug-in receptacles don’t offer the kind of resistance one might expect if such bumps existed.

#2 and #3 are closer to the mark.

Apparently the holes are optional according to a standards body called NEMA (https://archive.org/details/NEMA-WD-6-2016/page/n24/mode/1up) and are generally intended for use in the manufacturing process.

“Hole in flat blade is optional, and is intended for manufacturing purposes only.”

See 13:30 in the video for an example.