Well, I don't know what we're counting here. Generally, if FOSS apps have notifications which one might perceive as annoying, they'll have a checkbox in the in-app settings, so I don't need to *block* them.
There is one scenario, where I've blocked notifications, which is when an app wants to run in the background, then it has to put up a permanent notification. I hadn't counted that, since that's an Android requirement.
Aside from that, IMHO it's pretty clear-cut whether notifications are either necessary or subjective or not a good idea, so apps with user interests in mind can get that right quite well.
I don't know about most painful, but my dad bought a phone many months ago and last week, he wanted to know how to turn on the flashlight on it. I was ready to edit the notification dropdown or give a five step explainer or whatever.
Turns out, nope, you just pull down the notification bar and there's a pretty obvious flashlight button right there. The problem is, you see, he did not know you could drag down the notification bar. There were dozens of notifications there.
I really cannot blame him either. I don't know what UX designer came up with just putting a bar at the top and expecting users to know that you can drag on it. But yeah, still, ouch.