There’s only 2 questions tagged 2d, yet that’s a very interesting subject.
So I challenge the community to post on this topic, and discover existing posts that can have this tag added.
For questions with an element that only has two dimensions: length and width. This tag is only relevant when describing something that is often not two-dimensional.
Questions with this tag focus on objects, spaces and other elements that possess length and width but not height. The spacetime-dimensions tag is often relevant.
If your fictional universe is two-dimensional, it is difficult to use the science-based tag. See the Physics SE discussion on the subject, which explains the problems asssociated with such a world.