I use whom whenever it is necessary and required. In my country, English language exams are taken seriously and we are supposed to use this pronoun correctly ; we always have to know and demonstrate the difference between who and whom.
I used to use whom whenever it's possible in communication. But I don't think that if you use who instead of whom it means you are using slang. Am I right?
Because nowadays people kept chasing time, so they want to speak economically and quickly by any means.
If you are a non-native speaker, it's better to keep using whom instead of who, or else we might get confused... Just my opinion though