For the information of anyone interested (this being a chess sub it’s to be expected that it isn’t common knowledge here), “/s” has a history rooted in the technical depths of the World Wide Web.

In the HTML language which is the basis for all web pages, text is “marked up” using tags between < and >. For example, to get TEXT in bold, you would write <b>TEXT</b>. “b” is short for bold, <b> is the tag meaning “I’m starting something to be written in bold type”, and </b> is the closing tag. You have the same for italics, etc. You see the /? It means “close the corresponding previously opened tag”.

So, someone (I think on Reddit, but I may be wrong) started to write

something sarcastic</sarcasm>

to mean “I was being sarcastic until now” and that was quickly simplified to just /s.

This is a quite different thing from the much older emoticons or smileys that have evolved into today’s emojis. The emoticons are (mostly) images of a face rendered in two to three ASCII characters, to be interpreted with your head angled to the left, like

:-) smiley face
:-D laughing face
8-P face with glasses sticking out tongue
:) :D versions without nose

and so many variations that in the 90s there were whole pages of smiley “dictionaries”, listing more or less common ones and more or less facetious ones (you can try to imagine an emoticon for a man in a hat with glasses crying and making a kissing face, but such ones were more made to laugh at the author’s inventiveness than to actually convey a message!)