Question new form validation rule

It sounds like it could be a good idea. If this is complicated, it could make more sense as a separate class, like Validation/CreditCardRules, or Honeypot - that one is intended to be used as a filter, detecting bot access without needed a form validation rule.

The CodeIgniter4 Feature Requests would be the "most proper" subforum, but this one will work too.
I closed your "issue" on github because it wasn't a bug or approved work package, which is what we use github issues for.

You also provided no details about what you had in mind, other than a "new form validation rule" that might be complicated. Form validation rules may not be the only or even best way to handle what you have in mind.

More details about what you have in mind, and how you see it working, are likely to evince some community comments, and to help determine if it is a good fit for CodeIgniter4 and how best to implement it. Submitting an unsolicited PR to the repo is another way to go, but you run the risk of a tougher audience (maintainers) and of not having used the most appropriate approach.