Mastery still exists, but the design is a little cleaner.

Before:
Arms got some passive damage per talent point.
Arms got some passive armor pen per talent point.
Arms got a chance to proc a swing per talent point, which is enhanced by mastery rating on gear.

Now:
Arms gets whatever it needs as passives at level 10.
Arms gets a chance to proc a swing at level 78(ish), which is enhanced by mastery rating on gear.

Mastery now means “proc a swing” for Arms and that’s all it means, which is easier to understand. (Source)