sgr
The northern runway 26R was still in use once per year for the Flugplatzfest Gatow festival (last event 2019, subsequently cancelled due to the pandemic) – see e.g. https://www.youtube.com/watch?v=OTzs7wRKkjc

In the aerial photos from 2021, the northern runway is still visibly marked as 26R, whereas the southern runway has markings with an X, designating a closed runway (a tag `abandoned:aeroway=runway" might be more appropriate for that one, as in TXL for example).

I'll post a link to this note on a changeset by an OSM user who did some work to improve the Gatow airfield tagging, maybe they have an opinion on this: https://www.openstreetmap.org/changeset/99330213