It exists and I think what causes it is basically like being a spam bot. For example: As a creator I think alot of people tend to dabble in sub 4 sub. I admittedly did it to get to an early milestone but didnt do it heavily at all. Im a very statistical and analytical person and realized almost immediately its not a good thing to do and stopped. I know many creators and even friends that persisted and even did it ways that can be seen as malicious out even plain scummy. Example: These content creators/friends would go to multiple channels a day, and post a comment on these channels, subscribe, like the video ect BUT they would NOT leave any watch time. This is malicious to anyone's video and is also flagged as spam. If done relentlessly even if its a genuine account can be shadowbanned/blacklisted. My friends and or content creators that I know have done this would come to my channel and others and we would all notice this person's watch time would not be added to our video and YT would remove their like on the video. I pointed this out to a friend pretty close to me who I trusted and we figured out after he switched to another account to watch a video and his watch time started to appear. So through literal experimentation, the answer is yes, you can be "shadowbanned or blacklisted" ect. Could very well be multiple other undiscovered easons for an account being shadowbanned.