Shadow banning is a practice many modern websites use to silence users without their immediate knowledge. Logged in users are not shown any signs that they are banned or blacklisted, everything appears normal. Other users and non-users do not see comments or posts by users who are shadowbanned but but the user does see them among the other posts and comments. This makes it appear as if the user account is in a good, normal state even though it in reality is not.
Levels of shadow banning
Many of the websites who use shadow banning as a tool have varying degrees of restrictions placed upon users who are shadowbanned. The link aggregation website Hacker News shadow bans users and website domains. All posts by users who is shadowbanned on that site are immediately marked as being "dead" and not shown to other users. Those same posts are shown to the user who posted them as part of other content on that site as if they are regular and "alive". Hacker News will also ban links to website domains. All links to a shadowbanned domain are, in the same way, immediately marked as "dead" and hidden from other users but the user who posted the link will see the post as if it is just another post among the rest of the content.
Twitter has tools that allow moderators to shadowban posts so they do not appear in trending lists or search resuts.
Sites using shadowbanning
How to check if you are shadowbanned
Most websites will simply shadowban by user account and not do anything special to try to fool someone who is shadowbanned into thinking their user account is healthy when it is not. Logging out and reloading pages with a post or comment from the user-account in question is typically enough to see if one is shadowbanned or not. It is quite possible to use a cookie to check if a regular user on a site is browsing it without being logged in and some probably do this. It is therefore wise to use a separate web browser profile (you can start Mozilla Firefox with
firefox -P ProfileManager to create multiple profiles in Firefox). Websites can present different content depending on that IP/subnet someone is coming from and hide shadowbans that way. This is not very common, no major site is known to do that but it is possible. Using a VPN or the Tor anonymity network to view sites from a completely different iP address may be worth it if you are in doubt.
Simply asking a friend with a user-account at a site where you suspect you may be shadowbanned is a very simple way of finding out. That is your best option on those sites who require a user-account to see the content (Parler is one example of a site where you must be logged in to see anything at all). You could also create a new user-account of your own. Make sure you use a different IP/location and a web browser with no cookies if you go that route, some sites take simple measures to discover and link accounts created by the same person.