While it's true that platforms on the internet become more and more similar to each other, both in terms of functionality, lack of customizability, and design, it's not possible to allow unsanitized input anymore. There are too many ways to exploit input, not only with JS, but also with CSS, and trying to allow custom styles while preventing exploits is an arms race.

That said, social platforms not allowing users to express themselves through canvas and styles could be a matter of consistency and usability, while the myspace pages were a form of expression; they were certainly a pain to use and navigate through.