Since the S5 isn't two years old yet, the device still has quite a bit of life left. But right now owners on AT&T's network aren't getting Marshmallow. They're getting Android 5.1.1 as part of build number G900AUCU4COI5.
This may not be the most exciting update anymore, but it's a pretty decent jump from 5.0. You know, bugfixes and such. This OTA also brings Android for Work, and an Ultra Power Saver mode fix for encrypted devices.