My battery icon's remaining time stopped working a few months ago. I am on Arch Linux and have already asked for help there several times. Can anyone here help me resolve it or at least give me hints about what modules could be responsible for this? If I run acpi, I get the right remaining time, so I guess the communication between acpid and plasmashell are not working. Or is it powerdevil? I am not sure.

It's kind of disappointing that nobody's replied here because I just started having this same problem a few days ago in KDE neon. I posted about it on the KDE subreddit here -- the gist of it is that I can still get the time remaining from upower, but not from the battery status indicator.

I don't suppose you've got any leads in the interim?

Same here. The "time remaining" field on the battery applet has been stuck since several months now.
Originally it only got stuck with the standard linux kernel (probably around 5.0 at the time) but not with linux-lts (4.19.x).
Now it's stuck with both the long-term support kernel (4.19.75) and the regular kernel (5.3.1).

I am also getting this problem, basically the battery indicator doesn't update anymore. I am running the latest kernel version 5.3.7 if that helps.

This was posted in the wrong subforum, might be one of the reasons you didn't get any attention. Also did somebody file a bug report about this?

FWIW: all things Plasma related should go to the Plasma 5 subforum, except for Neon specific issues that belong there.