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
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
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.