

Well, first of all, it’s old people slang at this point.
And second, I don’t understand your outburst. You’re free to start a serious thread about criticizing Russia’s rhetoric, right next to the sarcastic thread that is criticizing Russia’s rhetoric.
The one who’s hard to understand is you, with your complete lack of punctuation. “/s” is extremely common and pretty much everybody understands what it means. If you didn’t know until now, no biggie, now you do and all is well.
And to answer your question, if a comment or a paragraph ends with an “/s”, then you can be sure the author was being sarcastic in that comment/paragraph.
If there’s no indicator, it could still be sarcastic or it could be serious, use your own judgement.
So no, not everything should be taken as sarcasm.