I disagree. In verbal sarcasm, there's often an equivalent, where the sarcastic phrase is said with a certain tone, or certain syllables or words are emphasized, to convey the meaning that that statement is sarcastic and not the actual intent of the speaker. That information is lost in written text, and something like /s simply creates a written equivalent. It hardly "ruins" the sarcastic statement when a verbal equivalent might be similarly blatant and the mark to signify sarcastic intent is only read after the rest of the statement anyway.

Just figuring out a sarcastic statement by virtue of that statement being absurd enough as to not possibly be intended seriously, does not work in situations where the statement is presented by itself without other context, and the assumption that "nobody would say that thing unironically" is false, because in such a situation, the sarcastic and non-sarcasic use appear exactly the same.

Further, having no standard for conveying sarcasm unambiguously would mean that someone who really did intend to say something like that unironically could simply hide behind "I was being sarcastic" when called out on it.