It's trivial now, with a limited number of characters, but you will see many many multiplayer games that potentially have like 30+ people in an area still skip doing IK locking.

The game I'm building with a friend right now allows four players in online play; doing IK locking (both for foot placement and—as this game has parkour stiller movement—hand placement) and slope deformation is a no-brainer; it doesn't have a prohibitively high computational cost and makes everything look so much better.

If I had an environment where I had the potential for 50+ characters in an area? I'd be a lot more careful about making that choice, as doing more than very basic dynamic IK (e.g. the sloppy and imprecise slope deformation many games use) would have implications for the minimum system requirements even today.

When the ARR engine was put together, dynamic IK on that scale would've been an absolute non-starter on the PS3.