Tag wiki excerpts are plain text, they allow no formatting whatsoever. In particular, there are no hyperlinks in tag wiki excerpts.
Including a hyperlink in a tag wiki excerpt is a common mistake. It would be easy to block them server-side.
If a suggested edit or direct edit for a tag wiki excerpt includes the substring http://
or https://
, please reject it outright, with a clear message (something like “Hyperlinks and other formatting are not supported in tag wiki excerpts.”).
(The tag wiki excerpts for url
and http
on SO and SU don't include any hyperlink, so I think it's safe to say that no excerpt needs to include text that happens to look like a hyperlink.)
This could be generalized to other attempts at formatting, but the risk of false positives may be too high.