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.
A while ago I proposed having a spam report system that junior admins and above would have access to. Shadowbanning could also have something like this implemented where everyone who is junior admin and above can see it and discuss it on a private sub or something before a users is shadowbanned. Hopefully it will reduce the amount of legit users being shadowbanned in the future.