At least some forums are discouraging that now because it obviously won't hold up in court and provides a false sense of security, but I imagine some people are still attached to it.
They are not.
Cops are demonstrably not required to tell you they are cops. The idea that they need to was always ridiculous misinformation.
Meanwhile, as far as I'm aware, SWIM has not been the lynchpin in a court case yet, so it's hard to say one way or the other if using it as legal cover is actually a gross misconception.
It doesn't work that way. If you try to buy 50 grams of coke off an undercover cop, no jury on earth is going to believe that you thought you were buying Coca-Cola. You're not going to get away with using a slang term universally understood to mean "me" and claiming it meant "someone else," either.
And then they typically shorten the whole thing to "I think you should talk to a lawyer."
(IANAL though, check with your local laws and all.)
It's not about covering your ass, it's about trying to be a decent guy and not accidentally mislead people.