You can. I've even seen intentional 4:3 used as an "80s" signifier.
Quick googling suggested that square video under 3 minutes will be automatically classed as "shorts", which much of HN hates and may never have seen.
Not YouTube but regarding the period signifier:
Wes Anderson's "The Grand Budapest Hotel" has every section of the movie shot in the time appropriate aspect ratio.
Not YouTube but regarding the period signifier:
Wes Anderson's "The Grand Budapest Hotel" has every section of the movie shot in the time appropriate aspect ratio.