AFAIK the pure platonic ideal of OOP (i.e. the thing that Smalltalk and Self do) makes no mention of subclassing — that’s bolted-on brain-damage that’s become conflated with OOP.

The way a “real” OOP language would do subclassing, would just be composition: wrapping an instance of the “parent” and proxying messages up to it.

If such a language wanted to “support inheritance in the language” (i.e. put syntax sugar around that wrapping), the result would probably look something like Golang’s anonymous embedded struct fields — where embedding a “parent” object of type X, gives your wrapping object Y default proxy methods to call the X instance’s methods of the same name; and default proxy getters/setters for the X instance’s fields; both of which you can override as you please / are implicitly shadowed by methods+fields of the same name declared on the wrapper type.