Has aspect ratio ever been used this way before ? Spoilers
Not so much the square image, but in the way the aspect ratio reflects/compliments the film by extension.
It seems so intimate and closed in, then magically opens during a time when everything is finally going right, only for reality to come crashing back down, or in this case, closing in again (with being served and reminded of Steve's fire and the resulting injuries.)
Later it opens up again, when we see the montage of how Diane idealizes/dreams/imagines a future that might never be. She's lost in this thought only for the aspect to once again close back in, as she comes back to reality, and remembers what her intentions and next step with Steve are.
I don't think I've ever seen a director play with the aspect ratio in quite this way before, yes there are many experimental films out there, but never have I seen just the size of the image used as symbolism before.
I'm tempted to say it's groundbreaking, regardless, it was an ingenious choice on Dolan's part.