A primary style would serve this feature better, I think, as it’s a more advanced use case and so more confusion to people reading code where two styles of a seldomly used API are in use. My demo also allows accessing the static func on the type directly, but at that point it’s easy to drop the type name, so it’s less of a separate style.
The main objection I have to lhs.isKnownIdentical(to: rhs) is that identity checking, like equality checking, should not treat the two sides differently. a.isKnownIdentical(to: b) should be the same as b.isKnownIdentical(to: a). And it is. But a?.isKnownIdentical(to: b) is not the same as b?.isKnownIdentical(to: a) and that is easy to miss.
Another reason against the member method is that it adds one more option to the autocomplete menu. This is more a feature that you use when you know you need it, and less one you should stumble over when viewing the autocomplete menu after typing .