Preserve linked text of an unresolved page xref
Upon encountering an unresolved page xref, Antora is dropping the linked text (aka content). This data loss limits the ability for a postprocessor to repair the link, such as to convert it to a link to an external site.
The linked text should be preserved in the content of the anchor, offset from the page ID by a pipe. In other words:
<a href="#">folder:page.adoc|Linked Text</a>
Keep in mind, even this is a temporary solution until Antora supports a proper callback for handling an unresolved page xref.