SQL11 introduced a couple of temporal features:
One of them is FOR SYSTEM_TIME AS OF which can be part of the table reference:
With this extension the users are able to query the table at a given time point.
HIVE-25344 also added support for this. It also adds another, non-standard clause: FOR SYSTEM_VERSION AS OF.
In Impala we should add both to support time travel for Iceberg tables.