Thank you anon. The specific example you gave was admittedly done on purpose. That at first he's bantering with Bucky and sort of slips into the accent while he's in the moment, but as things veer away from that familiarity and onto the topic of Elsa, he's back to his more normal speaking voice It's a head canon of mine that Steve's accent was heavier when he was younger, but that he out grew it as he got a little older (which happens to some people). Every so often it slips out, but mostly around Bucky. It's something I sort of noticed Steve doing in TFA. Every so often he'll get an accent when he's being snarky or dropping one-liners ("Nothin.' I'm just a kid from Brooklyn." for example.)
He still uses words like "kinda" or "gonna" even in WS, but a lot of people will abreviate or mesh words while they speak. But if it's happened in other threads where he'll drop things like "ain't" or the sentence structure is more 1930's street talk, then I did make a mistake. I'll be more careful about those slips from now on >>;;;
Thanks for the feedback anon, I'll definitely keep an eye of for this in the future.