Currently, the changefeed poller emits row updates at the ModificationTime of the table descriptor where the column backfill mutation has been completely applied. If the schema change was started when the table descriptor was at version 1, the table descriptor version at which the changfeed level backfill row updates are emitted would be version 4.

The above is correct in case of ADD COLUMN since descriptor version 4 is the first table descriptor version such that the new column is logically available. However, in case of a DROP COLUMN, the dropped column would logically get dropped way before this (at version 2 if the schema change started on version 1).

We should be emitting backfill row updates for dropped columns at the ModificationTime of the table descriptor at which they're logically dropped (which is version 2 in the above example).