There are some older questions about this (2013 for 4.0: How do I get rid of the "Battery fully charged, unplug charger" popup screen? ; and 2010 with a 2016 answer: How to disable battery full notification sound? ).
Is that really the state of things? My phone, Cat S61 with Android 9, used to alert me perhaps once every half hour about "battery full, unplug changer" and respected my 11pm to 8am do not disturb. That was mildly annoying if I wanted to sleep in past 8am, but not terrible. Recently, however, it has started alerting perhaps every 30 seconds and it will alert even if the battery is not 100%.
All of the apps suggested as fixes in the previous answers are either 404 links, no link provided and too generic to identify (looking at you "Sound Manage"), explicitly don't work, like Tasker, or require rooting (AFAIK, no root available for my phone).
There are no options in settings for setting volume on other alerts, and only turning down ring volume seems to work, at the cost of it vibrates every thirty seconds instead and pops up an onscreen alert if the phone is unlocked. I have searched for "battery" in settings for everything possibly related.
Turning the phone off to charge does work, but is a drastic solution. Is that really it? Has nothing improved in four years?