In how far is Hipparcos data still being used? I remember being excited when ESA launched Hipparcos in 1989 and was in operation until 15 August, 1993. Later, Gaia came into operation in 2013.
Reading What is the overlap between the Gaia and the Hipparcos catalog? I started wondering whether any of the Hipparcos data is still being used and for what. Could somebody please guide me with a concise explanation?
Reference

https://www.cosmos.esa.int/web/hipparcos/catalogues
https://www.cosmos.esa.int/web/gaia/data
The Hipparcos results can be used as an earlier epoch to improve the astrometry produced by Gaia. This was the linchpin of data release 1 (DR1), but less important in DR2 and DR3, because the astrometry of Gaia alone is so much better in general.
An exception to this is bright stars where much data is still missing in DR3, but can be found in the Hipparcos catalogue.
Apart from precision-based concerns, the earlier Hipparcos data, combined with Gaia, can be used to look for changes in proper motion: i.e. accelerations (see Brandt 2018).