It depends how obvious, but honestly I don't think it's that bad. Some non-native English speakers might not get sarcasm, and it also depends a bit on culture. British people are a fair bit more sarcastic than Americans and it's not always obvious in text.
To be fair to the /s we do live in a world with flat-earthers and antivaxxers. These days it can be exceedingly difficult to tell between the smartasses and dumbasses
I used to be a big proponent of “/s”. My rationale was that reddit was a worldwide site, and it wasn’t fair to exclude people who weren’t fluent in English. Plus, I didn’t want to live up to the stereotype of the selfish, arrogant American.
Now, I just don’t worry about it. Over time, I realized I could say something obvious and seemingly non-controversial, like “the sky is blue”, and people will disagree anyway. If somebody a thousand miles away takes a sarcastic statement literally, and decides that I’m an asshole, then so be it.
I don't care too much whether people mark their sarcasm or not.
I usually do because I've seen enough fucked-up takes back at the other site (that I confirm they truly believe from their profile) that I don't want people misunderstanding me, especially when my comment would begin with a sarcastic tone.
To be fair, that stemmed from an inability to sense tone through text. It doesn't matter if something seems obviously sarcastic to you, other people don't know you, or your sense of humor, and are more likely just to think you're an asshole or blatantly wrong