There was a plugin for vim that would blur out everything in the current file except for the function your cursor was on or in the body of. I've googled every combo of query that describes that and looked through git commits of my old vimrcs but can't find it anywhere!
If you would scroll down or up into another function/body it will "reveal" that and blur everything else. It would also center with horizontal margin.
Regardless, is there an edit to zen mode or a plugin that could achieve this? And if not the blur effect, then is there a way to limit the characters visible to a number?
Something like the image halfway down in this article.
Finally found the answer: it's a combination of two vim plugins,
goyo.vimandvim-zenroom2. When activated this is what it looks like: