Shadowban users posting obvious spam instead of just banning them. Banning accounts just leads to more spam accounts, and we have no account creation restrictions in place. If they are shadowbanned, they might not create new accounts and they will carry on posting with their shadowbanned spam accounts.
If we are going to implement this, I think it is important to have a clear definition of what we mean by spam. I am not talking about people posting off topic things in the wrong sub, or people making a one off spam post trying to plug their youtube channel on /s/gaming or something, I'm talking about users like mallow and jacknight that are constantly posting stupid shit and not contributing at all.
Reddit's policy on shadowbanning is fucked up, but I think it could clear up any potential spam problems if it was used in the right way.
Edit: If this is implemented, I would suggest not putting this in the TOS or anywhere else. Don't let the shadowbanned spammers know that they have been shadowbanned, or it undermines the effect of shadowbanning.
I think it could be problematic for a couple of reasons. One is it's going to be difficult for new users to figure out where they can post to stop the posting restrictions on their account. Another one is there are going to be a shit ton of introduction posts flooding /s/all if we get even a small amount of traffic.
It's hard trying to come up with a restriction that is not going to be irritating for new users and stop spammers at the same time. Ideally, someone should be able to create and account in a few seconds and immediately start posting, which is why I am more in favour of shadowbanning persistent spammers rather than placing restrictions on posting. I wouldn't like to have anything similar to Voat's negative CCP restrictions either, because it's far too easy to use brigading to shut down someone's account.