The link above errors:

A field that was supposed to be an integer (for our purposes, a whole number between -2147483648 and 2147483647) was no

I'll fix the link.

I strongly support this feature though nonetheless. It could even be used as an indirect confirmation someone read the rules (you can ask a question about the rules) in addition to keeping spammers out.

I have added a very primitive version of this, with a novel twist, as a developer addon. This addon is running here. This is what is now in the docs...

[title="2"]Per-site Q&A / Probation / Shadow-banning (advanced)[/title]

There is a developer addon, [tt]antispam_question[/tt], which checks the value of a custom profile field to see if it matches a pre-defined setting. If it does not, it puts the member in the Probation usergroup only.

You could then then configure your forum permissions so there is only a single forum these members see, that normal members don't. This effectively works as a shadow-ban. You can then move people out of Probation manually if you need to.

The developer addon hard-codes the CPF ID being checked against, and the expected value, and the ID of the usergroup to put failing members in (the default Probation usergroup). It would be fairly easy to customise with minimal coding skills.