PropertyTypes are all sealed by definition. ObjectTypes and VariableTypes are never sealed by definition. Are you asking that Object and Variable Types be defined that can be sealed?

The PropertyTypes may be sealed by definition but this is only defined through text in the specification, not through a machine readable information.

I am asking for a general feature to indicate that a type is sealed.

If we have this feature, we should use it also for the PropertyType.

A "nice request". But IF the intention would be to be backwards/forwards-compatible, it is close to impossible to retrospectively add this to any type. The PropertyType might be the only possible one where this could be marked (like... if I would nitpick, I would need to very carefully check would a theoretical subtype of PropertyType be allowed; probably not). No other existing type could "ever" be sealed, as there could be already existing subtypes, which this change would break.

Like, it might be nice to have, but this would have been something that would had to be in the very first release. Or alternatively this can only apply to new types. But then, IF there IS an old type (that I missed) that is "effectively sealed", it would mean that still cannot be marked as such, and then there would be a rule for all other (new types). Thus, a person reading a future spec version would probably make an assumption that that old type is not sealed, when it practice it would be.