Except it’s not listed as slang in the definition you’ve provided at all, is it? It’s listed as nonstandard, and in my personal, entirely subjective, holier-than-thou opinion, people that use it, rather than the actual word with which it shares an identical meaning, make themselves look stupid. I find (because I point out the ridiculous decision to use this word in real life) that 1. people don’t typically realize it’s stupid until it’s pointed out to them that it’s nonstandard and has no discernible difference from “regardless” and 2. that people that do realize this tend to agree with me, in my personal experience. Double negatives are not your friends. YMMV, but if you have friends that regularly say this, they probably aren’t as smart as they think they are.
Also, to be abundantly clear, “irregardless” is objectively not slang.
https://en.m.wikipedia.org/wiki/Irregardless