Oh yes, you are able to create an instance of

Class Your.Page Extends %CSP.Page
{
Property Name As %String [ InitialExpression = "Joe" ];
}

class because the inheritance goes as follows:
Your.Page <-- %CSP.Page <-- %Library.Base <-- %Library.SystemBase
and the %Library.SystemBase donates you the %New() method.

My guess is, because abstract classes (%CSP.Page is an abstract class) can't be instantiated, your subclass lacks the generator methos for property initialisation.

But there is a simple solution:

Class Your.Page Extends (%RegisteredObject, %CSP.Page)
{
Property Name As %String [ InitialExpression = "Joe" ];
}

Well, the world is right again

set page=##class(Your.Page).%New()
write page.Name --> Joe

Ha ha ha 😂, that's a big mistake. Those are old functions (for even older applications, maintained for backward compatibility only)  in the mean time all replaced by the $list...() functions.