There is no strikethrough button on the edit bar. You can still do it using <strike>test</strike> or ~~strike~~ or even <del>strike</del>. But this is highly undiscoverable without going and searching the web (and even then, you need to know that Unity is running Discourse for useful results, and that’s not obvious to most users).
Hopefully at least now they can search the forums and find this result.
It’s hard to believe there are buttons for much less frequently used things like tables and inserting the date/time, but there’s not one for strikethrough.