My guess is that the interpretation of html tags was scrubbed out to prevent some kind of injection exploit.
Strikethrough
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For example, <b>bold</b>, <i>italic</i> and <u>underline</u> all used to work as well, in fact it was the only way to get them to work on an iPad which doesn't display the control bar on the text entry box (it did, breifly, but I think it was removed because it was crashing safari).
Yeah, it stopped working at some point.
There's still options like this one:
http://fsymbols.com/generators/strikethrough/
S̶t̶r̶i̶k̶e̶t̶h̶r̶o̶u̶g̶h̶!̶