My keyboard backlight works!
About a month ago I switch to FreeBSD on my laptop - a Lenovo Thinkpad T470. After an evening of messing around, I managed to get all the hardware working with the exception of the keyboard backlight. I resigned myself to live without this feature if necessary, and figured I could always add support for it myself. It works fine on linux, so the driver is likely open source and I can use it as a reference.
Well it turns out that won’t be necessary. A quick search for “freebsd thinkpad keyboard backlight”
yielded this lenovo support page
as the first result, which has nothing to do with FreeBSD, but did solve my problem.
Fn+space toggles the keyboard backlight on thinkpads, and it seems to be supported out of the box
(or possibly due to enabling the acpi_ibm module).
So that’s what that symbol means.