Ah, actually, don't turn on debug and don't edit the full file.
Instead edit js/xenforo/bb_code_edit.js.
Find:
Code:
c.push(["bold","italic","underline"])
Replace:
Code:
c.push(["bold","italic","underline","deleted"])
And this is all assuming that I have remembered correctly that the strike through used to be next to B I U...