I've here an extension/hack making extensive use of the LinkBegin hook. It seems, reading 1.28 release notes, that it has been deprecated:
... In addition, the LinkBegin and LinkEnd hooks were replaced by HtmlPageLinkRendererBegin and HtmlPageLinkRendererEnd respectively. See docs/hooks.txt for the specific changes needed for those hooks.
But neither the old page tells anything about it, neither the new hook one exists. It seems that some parameter was changed (reading code), and the docs/hooks.txt may be also wrong.
This applies to both LinkBegin and LinkEnd.