It is used in reviewing documents. Like http://criticmarkup.com/ . Perhaps we should consider an extended syntax dedicated to reviewing documents in commonmark that would include the strikeout.
In critics markup, they used {--striked out text --} to do the subtraction. Since {} will be used for consistant attributes, we could try [--striked out text --] instead.
Or alternatively >>--striked out text --<<
Brainstorming Reviewing Markups in future commonmark: (edit: moved to Reviewing Markups )