How do you do the strikethrough in the UCP?
Did you try this? (See below):
<s>Example Text</s>
Example Text
Can you provide a link to the page you tried it on?
Note: it seems to work for me (see this page for more info). Perhaps you didn’t close the HTML tag?
Which wiki is this on? Is it a UCP wiki? If so, were are you trying to use the strikethrough? If you are talking about Discussions or any of the other discussion features, you can't use it in those posts. If you are talking about regular wiki content or posts on a legacy discussion feature, then it should work.