Beatboxing […] is a form of vocal percussion primarily involving the art of mimicking drum machines […], using one’s mouth, lips, tongue, and voice.
Beatboxing is arguably an edge case between vocal and instrument performance (as mentioned before), but I’d say it’s first and foremost an instrument performance. Like with body percussion at it’s core beatboxing and other forms of vocal percussion is an act of performing percussion with the human body. Most good beatboxers will optionally include vocals in their performance, but beatboxing with just vocals is not beatboxing - it’s just humming. So I’d say beatboxing should be an instrument.
Percussion > Body Percussion > Vocal Percussion > Beatboxing.