It's possible to
strike a line in chat by surrounding it with triple dashes:
---Struck out text---
However, that's not specified in the chat FAQ. I think it should be added.
Meta Stack Exchange is a question and answer site for meta-discussion of the Stack Exchange family of Q&A websites. It only takes a minute to sign up.Sign up to join this community
Okay, I've added it to the "How do I format my message" section (which also appears when you click "help" in the chat room).