A shame indeed, they were only a few decades too early! The support for "Time Varying Data" gets discussed briefly in this 1995 paper by Stonebraker "The Design of Postgres"
> POSTQUEL allows users to save and query historical data and versions. By default, data in a relation is never deleted or updated. Conventional retrievals always access the current tuples in the relation. Historical data can be accessed by indicating the desired time when defining a tuple variable.
> [...] Finally, POSTGRES provides support for versions. A version can be created from a relation or a snapshot. Updates to a version do not modify the underlying relation and updates to the underlying relation will be visible through the version unless the value has been modified in the version.
I once spent some time trying to find a way to do bi-temporal versioning in Postgres.
The only thing I found was a half-dead abandonware external project and an associated presentation PDF from some conference the author once spoke at.
I was unaware that they previously had some form of temporal queries and deprecated it. That is a great shame.