A question about aspect ratio on youtube, Does it care? or can you put whatever aspect ratio you want, I guess my complaint is that I don't see nearly enough (none) square video on the site.

YouTube, as well as any decent player, plays any aspect ratio video, even portrait mode.

As an uploader you should never add black bars (if they are in the source, crop them out before uploading) and of course never distort the video. This ensures the best playback experience for all devices.

AFAIK youtube will stretch the player window to match the aspect ratio of the source media, lots of cinematic content that's a wider than normal (21:9 I think?) ratio that youtube adjusts the player window to fit around without black bars.

They won't ever squash or stretch video though, so this means the original uploader stretched the 4:3 content to 16:9 at some point before upload

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.