Issue with struck-out text

[s]This text should be struck out, but it doesn't work.[/s]
This DOES work, for some reason.
Also, why are there both "stike" and "strikeout" options if they both do the same thing?

"Should be", according to who? As far as I know, phpBB doesn't come with strike-out tag enabled, so the "strike" and "strikeout" tags were probably added by a mod of this board at some point. No reason why "s" couldn't be added as well, though.

It doesn't does work now.

HTML5 retconned a lot of old presentational elements, such as <b>, <i>, and <hr>, to have semantic meaning. The <s> element, for example, "represents contents that are no longer accurate or no longer relevant". For example, "doesn't" above is "no longer accurate".

I doubt this will work... Oh yes, so it does.