>>Why is the strikeout tag not allowed in a message anymore? I tried a few times to cross out a mistake and get the message of the only tags that are allowed.
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>Most of the HTML tags are no longer supported. I haven't seen any explanation given, other than that they aren't.

To prevent injection attacks most of the tags has to be excluded.
I guess some of the common tags just didn't make the "inclusion" list yet. If we keep bugging Michel they will be allowed eventually.