Users can only know their comments are removed if they check from logged-out or another account, or if the moderators notify them by posting a reply or messaging (which some do, but not a lot of the time overall).
And either many redditors have never had their comments removed, or more likely, shadowbanning is apparently a lot more effective than people realized.
Pretty much all of reddit's moderation tools were created 10+ years ago for anti-spam purposes, and have hardly been changed since then. They don't work very well for how they're actually used now.
Most of the subreddits with active mods and communities have yet to stylize and reimplement their custom CSS, sidebar and links on the new UI (or can't due to limitations of the new UI), which means lots of people using the new UI miss out on key features that differentiate a subreddit from the rest of Reddit.
While your comment makes it sound like AutoMod just removes comments, what I think you're meaning to point out is that AutoMod can selectively hide all of a user's comments to everyone except for the comment author, which is the exact same as shadowbanning, and isn't just a simple comment removal.
A lot of subreddits use this feature now, too. Some automatically shadowban any user that doesn't have above a certain amount of karma, while others auto shadowban everyone and then only maintain a whitelist of authorized posters. If you've ever posted on a subreddit and gotten zero replies/votes, it's worth going into incognito mode and trying to view your comment from there. Chances are, your account is shadowbanned in that subreddit.