https://defs.ircdocs.horse/info/formatting.html

https://adiirc.com/changelog.php?v=0

4.1 Beta Build (2021/09/22 UTC)
Add support for strikethrough control character

(Full versions 4.1 and 4.2 have released since then)

TBD: find someone who knows why the horse docs doesn't think anyone supports ^O reset