Comment 1 for bug 1948018

Sure, it should "work" but as a practical matter sealing a standard-class achieves nothing. Its known placement in the type system will not optimize anything- not method lookup, not TYPEP.

Lest there be any doubt, search for :SEALED in the source code and you'll find that any logic surrounding the lookup of the class state applies only to structure-object and descendants.
Patches to alter that could be considered. Otherwise I'd just as soon have the compiler style-warn upon seeing the declaration, to the effect that the declaration is unsupported for the metatype. (it's nonstandard so there's no spec violation there)