The same thing that happens when any other L7 or 8 completes a major initiative. They find something else to work on. Many of the steering group members also lead multiple (in some cases, a dozen or more) k8s working-groups, but they presumably also have responsibilities related to their company (and possibly how their company uses k8s), so they'd focus more intently on those things, or perhaps they started some initiative while on the steering committee and continue to lead that particular thing even once off of steering.
The same thing that happens when any other L7 or 8 completes a major initiative. They find something else to work on. Many of the steering group members also lead multiple (in some cases, a dozen or more) k8s working-groups, but they presumably also have responsibilities related to their company (and possibly how their company uses k8s), so they'd focus more intently on those things, or perhaps they started some initiative while on the steering committee and continue to lead that particular thing even once off of steering.