If someone is able to get around a ton of banning techniques, they will definitely notice they're shadowbanned.

In some cases, I’m sure you are right. That said, the online communities which currently employ shadowban systems continue to use them. That tells me there’s a value greater than their implementation and operation costs.

There is no perfect tech solution to a human problem. But in my opinion, having access to a partial mitigation is better than no mitigation.