In (StackOverflow) chat, when I type
---something---
It gets struck through.
When I do the same on any SE site, it is not.
What I need to do is the terrible
<s>something</s>
and then it is struck through. Why does the triple dash not work everywhere? It's much more "Markdown"-y than the html-y <></>-stuff.
Three dashes for strike through is not part of the official Markdown spec. To implement this would require that either the core project is updated or SE fork the project - which SE are reluctant to do. I would guess that it's not been done because the benefit gained is not worth the implementation cost.
If you could come up with a compelling reason why the "Markdown-y" method of three dashes is superior to the "HTML-y" <s></s> then the team might consider it.
The team have probably got more than enough work developing and supporting the sites and Careers, so they aren't going to want to take on more work.
--- works now, and <s> does not. I'm guessing balpha's experiment stuck. (I found this old question, btw, because I can never remember how strike through works in chat. Figured I might as well post an update to this answer while I'm here.)