Is shadowbanning really a thing on other websites? And why? What's the point? Why not just ban the user?
Yes, it's real on Instagram, and maybe Twitter? A little history on the mechanism:
Shadowbanning also goes by Hellbanning. I'm not sure where it originated but early 2000s forums like Something Awful used the mechanism.
The problem with straight-up banning a user, especially if they're a motivated bad actor, is that it's easy enough just to create another account and continue doing what they were doing to cause the ban. Shadowbanning at least played a trick on the user so they couldn't tell they were effectively banned from posting. If the troll or spammer thinks nothing is wrong, they just continue their behavior and it goes nowhere, rather than seeing the ban and creating another account to circumvent it.
It's even funnier when they allow other shadowbanned/hellbanned people to interact with each other, just not regular users. Creates a PvP Hell World.
Nowadays, I suspect shadowbanning provides another benefit to social media: shadowbanned users still see ads. So if an unmotivated user who would normally just stop using the service once they were banned, instead that user now continues to get served ads in their banned state. Plus you get the added benefits described above to deter bad actors.