Insights I got from my own account: a) Reddit randomly decides some posts were spam and shadow-banned them O.o Content was all over the place, from links to link-free text posts. Weird. b) Low quality subs (/r/europe, /r/Germany, cryptocurrency subs) liberally shadowban all kinds of comments. All other places this shows, have comments/flairs about the removal.
Subs cannot "shadowban" users themselves. Either one banned and gets a notice, or one isn't banned at all. Comment removal initiated by moderators themselves is not "shadowbanning" - that's something Reddit's automated mechanisms (and maybe admins too) do. I think you meant "content removal without explanation" instead. I will say, however, that for about half a year now or even more, reddit's own anti-spam detection…
They can. It's now called a "bot ban" and used to be called a shadow ban. It's done by adding a user's name to the sub's Automoderator configuration.
> With a bot ban, some users won't realize they've been banned. [1]
There is more explanation on Reveddit's FAQ under What is a shadowban?
[1] https://www.reddit.com/r/AutoModerator/wiki/library#wiki_use...