Repro

See#2699 for an example of when this could be useful.

Rationale

I see this as a trade-off - turning such an option on would reduce the strictness of the rule (as far as I understand it,Readonly<> only guarantees shallow readonlyness, this rule checks for deep readonlyness), however, it would make it much easier to use in certain cases like#2699 or#2079.