Since I've mentioned spam posts - the number of posts caught by SD in the last month is MathOverflow: 45, Mathematics: 54; All time - MathOverflow: 899, Mathematics 1589.
And even if that was implemented, this would not prevent anonymous/unregistered users from posting answers.
The Mathematics site requires registration (for asking questions) - and spammers still mange to post spam also on Mathematics.
Mostly spam posts on MO are removed reasonably quickly (I'd guess under 10 or 15 minutes, but I do not know how to find precise data).
If the number of posts caught by SmokeDetector can serve as a reasonable approximation of the total number of spam posts, it's still under 100 questions per month - which is probably not that many.
It's quite plausible that many MO users do not know what to do when they see spam posts. I saw people interacting with spam posts as if they were serious posts, editing them, even creating (spam) tag.
Probably making a post on meta with an explanation how the spam posts are supposed to be handled might be a good idea.
According to this recent post statistics moderators on Mathematics in 2018 are: Question flags handled: 3,750; Comment flags handled: 13,884; Answer flags handled: 4,769
So according to 2018: a year in moderation the stats on MO are: "Question flags handled: 856, Answer flags handled: 830, Comment flags handled: 1,385."
Probably one expects as a typical scenario that somebody asks a homework/low-level question on MO and after they see that they are on the wrong site, they go to Mathematics.
This is an example of a user who asked first a question on Mathematics - since it was just a copy-paste of a problem statement (PSQ) it was put on hold. And then they asked on MO.
