The callbacks in property metadata are not typically public members on the defining type, so the value of this property is not important for most scenarios that just consume an existing dependency property's metadata. One reason this property is exposed is so that metadata classes can perform their desired merge logic if both base metadata and overriding/adding metadata specify a PropertyChangedCallback. The default merge logic for is to maintain all PropertyChangedCallback implementations in a table and call each of them, with callbacks established by the deepest class in hierarchy running first.
PropertyChangedCallback is defined in the object model as read-write. This is so PropertyChangedCallback can be adjusted after initialization of the PropertyMetadata object itself. However, once the metadata is consumed as part of a call to Register, AddOwner, or OverrideMetadata, the property system will seal that metadata instance and the properties are now considered immutable. Attempting to set PropertyChangedCallback once IsSealed is true on this metadata instance will raise an exception.