Wikimedia Bugzilla is closed!

I'm marking this as WONTFIX.

You can use \cancel and \cancelto (and soon \bcancel and \xcancel) to get diagonal slashes.

The ulem package (for strikethrough) doesn't work in math mode (it underlines instead of striking through) so we'd need that fixed before we even considered this. An adequate image can be created with:

  <math>hello\!\!\!\!\!\!\!\!\!\!\!\!\text{------}</math>

but I appreciate that's not ideal.