So? Users playing by the rules of the service get to use the service unhindered. A much better approach to shadow banning would be to make better rules and enforce them. Spammers don't want to jump through hoops, so if you implement rules based on bounces and spam reports you'll get the same result (less spammers) but without screwing over legitimate users.

This is how Mailgun and their ilk operate, and while it's annoying to get bitten by their rules (we forgot to warm up a mailing list once and got a temporary suspension as our bounce rate was too high) they treated us like adults, told us why our service had been suspended and proceeded to help us clean up the mailing list. If they had pulled some shadow banning BS we'd have just left the service as we wouldn't be able to trust that they're not messing us (and our clients) around.

Shadow banning works just fine for online forums and the like. It's a pretty terrible method of rate limiting though.