[hunter2_]

In spoken word, sarcasm in even the cleanest context always has the benefit of delivery paving the way for virtually guaranteed reception. A written indication does look silly when reception succeeds without its help, but guaranteed parity with speech seems like the winning maneuver.

[johnnyworker]

The whole point of sarcasm is to say something you don't mean, and people who know you or other facts well enough, can get a dopamine hit for being able to infer that. It's just an ingroup/bonding thing and carries no information or value outside of that. At any rate, adding a /s is like saying "just being sarcastic", might as well not be sarcastic in the first place.

[hunter2_]

I think what you're describing is a subset of sarcasm, or potentially a separate thing that often utilizes sarcasm (significant overlap). A friend of mine will often suffix such statements in speech with "if you know you know" in the event that the delivery alone isn't sufficient to trigger everyone's memory, and "IYKYK" in text.

Pure sarcasm has no such prerequisite, which means that delivery alone (tone, cadence, and smirk) is almost always sufficient. These give away the fact that sarcasm is happening to roughly the same extent that "/s" does, just that we have more experience with the former so the latter tends to feel too spoon-fed.