Starting from SERVER-61004 the vector clock is checkpointed before removing DDL coordinator documents. That change was aimed at ensuring causal consistency with the sharding catalog by persisting the config time so that it could be recovered on step-up.

That's not enough because the only way to ensure causal consistency is to persist the vector clock before exiting the critical section (aka before allowing refreshes to hit again the config server). Considering that, we should probably get rid of the changes introduced by SERVER-61004: persisting the vector clock is not an issue, but it's not useful either in that point of the code.