It aired as part of the Menagerie. And the illusion is taken from Pike's memories, not something made up by the Talosians.
But it did air! As part of the Patrick Stewart-narrated "A Star Trek Saga: From One Generation to the Next" that was screened to lead into the late-running Season Two of TNG. The whole episode of "The Cage" was part of the documentary. And "The Cage" was then made available to the regular syndication package for TOS.
But does a documentary count? If so, there's a behind-the-scenes documentary that features, in part, a scene between nuKirk and nuSpock speaking to each other in bad Irish accents right before nuSpock kamikazes the Narada in the Jellyfish. Because that aired in a TV documentary, is that canon?
Even if the entirety of The Cage isn't canon, the portions (majority) of The Cage that was display in the episode The Menagerie would be considered canon.
No. "The Cage" never aired AS a documentary. It aired - both alone and as part of "The Menagerie" - as a simple episode of the show, just like any other. Therefore its canonicity shouldn't be in dispute. As for catsuits/unitards: God help us. I don't begrudge anyone else the right to wear them if it's comfortable for them; I just know I could never wear something like that, as the image of my fat ass in a catsuit would cause a deadly stampede for the exits, plus it would just be uncomfortable as hell for me (not to mention embarrassing).
In the future no one is fat unless they want to be. Harry Mudd clearly embraced his rubenesque qualities, probably helped him seem humble and trustworthy as a purveyor of fine goods. But for anyone else, and this probably only increased in the 24th century, you can modify your body any way you want. Look how people turn into other species and then back again in a day! Getting buff and losing flab would be nothing compared to that.
No, documentaries are from a real-world perspective. But "The Cage" had its TV debut as an entity unto itself, tagged onto the end of a documentary that introduced it. And "The Cage" did air after that, in other places, as part of Paramount TV's syndication package for TOS. It was tricky for stations to schedule, due to its irregular length.
I think it all comes down to a logistical problem. How do you get out of those things in a bathroom emergency situation? Do future bathrooms accommodate timely wiggling out of catsuits?
Many years ago, a friend of mine wore a bodysuit as the basis for her costume (she had other items attached to it). She hadn't considered the problem of bathroom logistics when designing this outfit, and discovered that to use the bathroom, she'd have to take most of her costume off.
Based on how the TNG Season 1 uniforms were constructed (I own one), use of the bathroom (especially when going #2) is accomplished nearly nude. Hardly efficient.
I can't even begin to imagine how you'd put on such a costume. How is it fastened together? When our Star Trek club designed our own uniforms back in the late '80s, we did an original design that combined TOS, TNG, and TWOK plus a couple of original elements (it's a nicer look than it might sound). They were two pieces - definitely easy to put on and take off.
The TNG first-season costume was a single-piece jumpsuit. It has a very long zipper which runs from the low part of the crotch all the way to the top of the neck. There are two small hook closures semi-concealed at the top of the zipper, which bring the ends of the neck piping on either side together. To utilize the restroom, one must undo both closures and unzip the jumper all the way, and disrobe from it until it's around your ankles. That can get chilly.
They should have consulted fan costume and SCA costume designers. It's a basic thing the experienced costumers drill into people: never design anything that prevents you from using the bathroom!
Not uncommon. Andrea Weaver told me that on TOS many of the actresses were sewn into their costumes and had to have a seam undone in order to visit the necessary.
So, no worse than mechanic's coveralls. The "catsuit" I have (clubwear) has a small zipper that starts at the throat, goes down, under, and almost reaches the small of my back, just for the reason being discussed.