At least if my post is deleted I can tell. But when companies artificially limit the reach of content they do not like, you might never know.
Facebook openly admits to doing this. They are playing god by curating which information is worthy / unworthy of being seen, as well as presenting a warped view of reality to their users.
You may not even have been aware that the above is a thing; most people aren't. But in my experience it accounts for a large fraction, if not the majority, of moderation actions against real human users on Reddit.
Shadowbanning is very similar but IMO less insidious since it's a lot easier for human users to notice. I'm not sure if per-subreddit shadowbans are a thing, but I lean toward no because it has not happened to me (that I'm aware of).
Reddit as a whole does have shadowbans but at least as far as it is public knowledge that functionality is restricted to the admins. Perhaps moderators of large "default" subreddits get special tools though or perhaps they can call up the admins to do it for them.
Is shadow comment deletion not available to AutoModerator? My own experience suggests otherwise, though not conclusively.