Add a custom property type that just inherits from PropertyLongString and add an EditorHint attribute:
public class LegacyLongString : PropertyLongString
Then you can add properties of this type.
If I create a page property in admin mode with the type "Long string" it displays as the "Short string" property in the form edit view.
I have migrated from CMS 6 to CMS 7.1 and don't have any page types defined in code.
Is this a bug or not? In case it's not a bug, how do I solve it?