Dropping any type of column from a transaction-time or bitemporal table requires the NONTEMPORAL privilege on the table, and the NONTEMPORAL qualifier to ALTER TABLE must be used.
Temporal columns cannot be dropped if any constraints or a join index or a trigger are defined on the table.
Temporal columns cannot be dropped if the table is partitioned on the temporal columns.
When a transaction-time column is dropped, all closed rows (all history rows in the transaction-time dimension) are deleted from the table.
When a valid-time column is dropped, all the rows that are no longer valid (all history rows in the valid-time dimension) are deleted from the table.