Never heard it until I read that thread. Lived in Cleveland area all my life, except for a few years in Colorado and Georgia, but then I didn’t hang with the locals unless they quilted, so slang word for police didn’t normally come up!
I have heard it quite often but not nearly as far back as some have. Until recently the warning cry that the police are here was “5-0”. That seems to have been replaced by po-po. Not an improvement in my opinion.
Still haven’t seen an answer to the OP’s question, and I’m curious. Is it a term of derision?
I can never read that word to myself without mentally hearing it through James Cagney’s voice.
How is it pronounced?
poe-poe? pu-pu?
(I’m woman of a certain age in a smallish Wisconsin town. It takes many decades for anything to find its way here).
I haven’t heard it as derision, just a nick-name. Like I said it used to be 5-0. Which I found amusing because I bet almost none of the teenagers who used it ever saw the TV show.
It’s from po-lice which makes it poe-poe.
Heh. I grew up not even in, but outside of, a very small Wisconsin town, and I remember hearing po-po in middle school, which would have made it somewhere between 1994 and 1998. Probably 1996 or so. It’s been around for more than a decade in Wisconsin, I’d say.
Not how I’ve heard it. It’s basically used to call attention to the fact that a cop is nearby and better slow down!
I wouldn’t call it a term of derision…but I wouldn’t use it in front of the po po either.
In my experience, it is not derision per se, but it would generally be used (non ironically) by someone who is somewhat hostile to cops or is generally happier when they are not around - it is not a term of endearment - Cluricaun was pretty spot on with his comparison to ‘one-time’.
I too have heard it since the early nineties, and i grew up in the suburbs of Milwuakee, WI.
I’ve only heard it in the last two months or so, but agree it’s a black/ghetto coinage.
(High Fives fellow Bay area resident.) 
Just thought I’d mention that where I work, there’s a fellow who is, shall we say, familiar to the prosecutor’s office. He goes by “Po Po”. The reason, apparently, was the frequency with which his friends would shake their heads and say, “Po’ po’ Willie got busted again.”