If the deer darts in front at the last second it will try to swerve around it, but maybe not too aggressively. Similar to if a car intrudes in your lane. But ya, if its standing in the road in front of you it will just brake.

Edit: Looks like someone did a video of it a couple days ago of swerving to avoid an inflatable dummy someone throws at it, so this is what I'd expect for a deer that jumps in front of it. Some pretty good speed reactions in there.

https://www.youtube.com/watch?v=9KRr9Of46MM

https://old.reddit.com/r/SelfDrivingCars/comments/1u69k79/fsd_reaction_time_testing_45_mph/