From what I can see I don't think there is a way to turn it back on. I've been through all the settings I can think of that would have anything to do with a slow charging notification. I think your best bet if you want it back would be to factory reset your phone. But with doing so you would have to choose not to have your settings restored since I believe this would be considered a user setting.
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.
@Ewclaw try to go to settings and apps and look for settings and clear data and cache and also go to notification and see if all turn on. I dont know if this helps but worth a try.
If all else fails open system settings apps and reset the app preferences. A pop-up will display describing what it will reset. It's a pain to put everything back the way you like it but a little better than a factory reset.