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'm not sure how well it would work.
Once the spammers are aware of shadow-bans, it is not hard for them to check. It could be scripted easily.
rEddit found that it did not work on spammers, after the spammers found out. But that it did work against genuine users. I'm not sure we need a tool like that.
I think at this point, mods can keep up with the spam. Later, maybe something like a minimum of 3 phuks to be able to post. 3 would be simple to get with an introduction post. But that would wreck the spammers.