Shadowban is not perfect by all means, but it's still a good deterrent in my experience.
Also, with tethering it's really easy to circumvent, without needing a VPN.
[1]: IIRC, the whole Laos only has a /32 subnet… yes you read it right: a single IPV4 address for end entire country. And many country only have a few /16.
And regarding third world country, your idea doesn't prevent them to access the website, but they will access a site where the shadowbanning feature is pretty much disabled, which could lead to the proliferation of trolls or spam targeted at this specific country.
(Apologies if I'm getting that number wrong. I don't do much with subnetting.)
[1] https://en.wikipedia.org/wiki/List_of_countries_by_IPv4_addr...
It's useful with the more sophisticated class too, though. If they have to start fresh with new accounts, it slows them down and makes what they're doing more obvious to the community.
And maybe the malicious/non-malicious ratio is low enough to make this method efficient.