Shadow banning is when a social media company, admin, or moderator hides certain posts from everyone except the original poster. Posts affected by shadow banning are no longer shown to other users in their respective social network timelines and may also be hidden from search results.
What is a Shadowban? | Untangled
In all cases of shadow banning, the post creator is completely unaware that their posts are being hidden from other people. They are not notified of the change and usually only become aware that they have been shadow banned when other users notify them that they are not seeing their content or when they notice a significant drop in post engagement or views.
In extreme cases of shadowbanning, a user's entire account, including their profile, can be hidden from a social network's users.
X's official stance on shadow banning is that they don't do it to the average user. However, the company has also confirmed that they hide tweets from accounts that have been muted or blocked by a large number of users or that have a history of guideline violations.