The sole feature addition to Wayland-Protocols 1.3 is the addition of the tablet protocol support that's been worked on the past few months by Peter Hutterer. This tablet protocol support is for the graphics/drawing tablets (e.g. Wacom tablets), not to be confused with say your favorite Android mobile tablet.
The tablet support for Wayland has come along well and the associated work within libinput.
The Wayland-Protocols 1.3 release announcement can be read on Wayland-devel.