Constraints can also encourage creativity, because people have something to push against. Larry Niven (science fiction writer) claims that constraints make a story (although he arguably writes puzzle fiction; and puzzles are defined by constraints).

I've seen a similar idea in niche marketing, where you specialize on a segment instead of spreading yourself thin to please everyone. Does this promotes creativity, or does it just focus it? I think the focus makes it engage with reality more, and that this does promote creativity.

This is really how content constraints affect content (not how medium constraints affect content.

Typewritten essays were very constrained in their presentation (especially in the form of the manuscript that the author worked with), including academic papers, such as Einsteins.

PS: maybe I misunderstand, and by "content" you mean the presentation that people create, not the text.