IRC pings don't use ICMP, but that could work if IRC clients would repeat data received without escaping first. Or if unaware users saw it come over the channel and tried to type it themselves.
Considering a CTCP request is just a PRIVMSG with ^A wrapped around the "command word", and a CTCP reply is just a NOTICE with the same ^A thing, you can make them "say" just about anything. It's unclear why an IRC client would need to worry about "escaping" +++, except if it's specifically been designed for people with bad modems.
Those of us with good modems back in the dialup days just laughed at this insanity. Hayes used to put "+++AT" in their press releases after a certain point just to trip up any noncompliant systems which may have passed it along.
Sorry, I should have been more clear. The modem could also guard the sequence by requiring interstitial pauses, but I believe this was patented by Hayes in the 80s.
Indeed it was, as the referenced Wikipedia article notes; Hayes charged $1/unit for a patent license. As soon as the primary application of modems became Internet access, IP encapsulation protocols like PPP could have worked around the problem, but, AFAIK, never did.
