Hi All !
Standard BBcode supports text strike through. 
However PHPbb seems to break this standard, and allow only bold, italic and underlined text.
It should be: (BBcode vs. HTML)
I know it is possible to add straight-through text via custom "BBCodes" in Admin Panel – but only for forums that I administer myself. This won't work for third-party project forum, that I contact with.
This feature is a standard BBcode, and must be supported for PHPbb as well, just like bold or underline text.
Tested with PHPbb 3.0.9