Shadowbanning is reserved for spammers and for new accounts that show signs of abusing the site, e.g. by trolling or egregiously breaking the site guidelines, or if there's evidence that they're related to past accounts that have been abusing the site. It's not a good idea to use explicit banning in all of these cases (for different reasons).
Here are some longer past explanations:
https://news.ycombinator.com/item?id=23686672
https://news.ycombinator.com/item?id=21288858
https://news.ycombinator.com/item?id=20666742
https://news.ycombinator.com/item?id=20289994
This system has drawbacks—banning new accounts means guessing—by definition, there isn't a lot of information to go on—and we guess wrong sometimes. But we try to correct those errors later. Btw, if anyone notices a banned account that shouldn't be banned, please let us know at hn@ycombinator.com—we've unbanned many accounts that way.
In fact I'm just working on a ban review system that will make it much easier for mods to track banned accounts that are still posting and unban the ones who are making good contributions (whether because we guessed wrong when banning them, or because they've reformed since having been banned).