Maybe I'm late to the party, but YouTube has made some big changes in its approach to player size and aspect ratios. On desktop, and depending on browser, no more black bars. Looks to be a concession to vertical video, which looks a lot better without the bars. Also, the player size adjusts to the aspect ratio. I first noticed the changes today in this France 24 square video. On Firefox and Chrome, but not Safari, there are no black bars when one watches the video on YouTube's site. On this forum, you have to click on the YouTube logo to move to YouTube's site to see the effect. In all three browsers, the size of the player is significantly larger than for 16:9: Similarly, recent vertical videos, in Firefox and Chrome, appear on YouTube's site without bars: Note that ads before a video appear in their original aspect ratio. P.S. France 24 is France's English language news broadcaster. I have no idea who Billie Eilish is, but her video was "YouTube recommended" and evidently she's new and very popular.
Yes, the bars will remain if they are part of your uploaded video. Apparently, YouTube started introducing this at the beginning of August. I think that the France 24 video (they've also uploaded a Part II video) was done quite deliberately as an experiment. On my 27" iMac, it is about twice the size of a 16:9 video. If the point is to get your attention, it succeeds, a point unlikely to be lost on others publishing to YouTube.
I'm a Final Cut Pro X user and custom project size projects uploaded to YouTube directly from it makes it easy. (4:3, Square, Vertical, Cinemascope etc.). Square video looks great on a desktop web browser because of the additional "height". Good news for all the anamorphic vids that should appear once users receive the new Moment Anamorphic Lens. It'd be awesome if there was a specification that allowed on the fly video aspect adjustment--go from square, to wide, tall etc on playback. Would be trippy. (Make your own "Brainstorm".) (A YouTube video playlist would probably simulate this "effect".)