I actually got this to occur the other day. First time I've ever had my phone tell me it was slow charging. From what I could tell, on my AOD/Lock Screen, at the bottom it said "slow charging". So while you may have the notifications disabled, it should be pretty clear that your phone will tell you it's slow charging; just as the same way it says fast charging. Just FYI.
i too have this issue. i would like to re-enable the slow charging pop up.
i use my note 8 for navigation frequently. my cars usb cable or charger sometimes acts funny and set my phone to charge slowly. i like the notfication since it lets me know i need to unplug and re plug the cable.
right now, since i cant tell. i wont notice for a few minutes when my battery drops even though its charging. so i will unplug and replug, but again, i still dont know if that fixed it the second time.