Seems that in recent times other clients decided to turn to use
\x1e as code for strikethrough.
No idea what the guiding document for irc code is, but searchengine seems to favour https://modern.ircdocs.horse/formatting.html#strikethrough, and I found related commits e.g. for hexchat: https://github.com/hexchat/hexchat/commit/55e4f1c42e8d1131b01659fad67db5d084780227
Given this seems the future, I propose to also switch to sending off formated text using this code, instead of the old.
Tested by chatting between old and new konversation instance, using "%Stest%S".