I don’t have a steam deck, yet, but my pixel also has this battery feature.

There’s some info in the settings, about the need to let the battery go to full 100% every now and then.

It shouldn’t happen that often though.

Once i had it go to 100% three times in a row, but other than that it’s always 80% and sometimes, often the first load after a reboot, it goes to 100%.