It would be interesting if someone could write a program that could diff two LaTeX documents and produce a change-tracked version showing insertions and deletions.

I think it's doable, but it's a serious project.

It's done and it works very well (in my experience) for comparing two revisions of a document: https://www.ctan.org/pkg/latexdiff

Plain old diff does just that.

I'm assuming that what you're actually asking for is a pretty GUI that pretty-prints diff's output.

latexdiff does exactly that: https://www.sharelatex.com/blog/2013/02/16/using-latexdiff-f...