Make the low population auto-shuttle unrecallable

[Roomba]

They mean the 'the shuttle is called because everyone's dead' auto-shuttle, not just any regular lowpop shuttle. I'm of two minds about this. On one hand, it's a pretty clear 'you win, round functionally over, please move on' signal to antags, and the shuttle being recallable kinda takes away from that. On the other hand sometimes a rando hops onto a lowpop round, immediately suicides and triggers the auto-shuttle, when everyone else alive still feels like carrying on.