One undocumented heuristic that I wish was documented better is that repeated submissions to the same domain, especially without discussion activity, leads to an automated shadow ban. This is just from what I’ve observed browsing /new with Show Dead turned on. There are lots of people submitting their own blog posts or repeated posts from their favorite news outlets who have been shadow banned. Vouching provides a mechanism for restoring posts of particular quality, but most of these people aren’t trying to spam, they just don’t know this unspoken rule.