I am trying to figure out what areas are not well documented online. That is, outside of books, paid articles, etc.
From the "basic" math, I would say that geometry is poorly presented online because of how cumbersome it is to type up fully and to animate the diagrams (for free! instead of publishing a book given you have the skill-set).
From the research frontier, I would think that Rough Path Theory seems to be poorly documented but maybe this is because it is relatively new? I was also thinking about Langland’s programme but it is a bit outside of my area of expertise.