This feature is currently optional and in beta. It can be enabled in your games within Roblox Studio by going to File > Avatar Settings > Movement and then changing the Abilities selection from 'Legacy Humanoid' to 'Character Controller Library'.
The sprinting ability does not use any new animations, it only increases the player's walkspeed.
Learn more:
https://t.co/4trV0624ES
Heres a look at the default crouching animations for both R15 and R6: