In PoW several nodes compete to be elected. If n1 does not get elected eventually nX will be elected. So PoW does clearly not need such a concept of timeslots.
In PoS lets say a node gets elected via random election. If that node is byzantine we clearly need a mechanism to achieve liveness. Every PoS algorithm that I've seen so far does introduce the concept of time-slots.
1) How do nodes reach consensus about the current time?
2) How do these PoS algorithms which use time-slots ensure that the time-source is not centralized?
3) How do these PoS algorithms which use time-slots ensure that the time-source is in sync?
4) How do you come to a consensus if a block was delivered in time? I mean even two non-byzantine nodes cannot possibly come to a consensus if an elected producer produced a block in time. Maybe n1 (honest) received the block in time and n2 (honest) has not received the block in time.