like Bluetooth, battery or battery level icons, you need to install Font Awesome. For instance, on Debian 12 (Bookworm). that's the fonts-font-awesome package.

In retrospect, it should be obvious. But for some reason it's mentioned nowhere in the Waybar man pages, your distro may not have a dependency to install Font Awesome automatically when installing Waybar (Debian doesn't) and it took me more time and effort to figure out that it wasn't installed than I wanted to expend.

So I figured I'd leave this here if someone runs into the same missing icons problem.