Matthias Göbel passed along information about the ESA Brave Days (http://download.nanoxplore.com password BRAVEDAYS-2019) and the GR740 User Day (http://microelectronics.esa.int/cgi-bin/userplain.php). Quoting Matthias:

About three quarters of the presentations mention that they do apply RTEMS. From this point of view RTEMS has to be considered as a standard in the European Space Domain. No other OS has deserved such an attention (despite those presentations may not be representative for the whole space domain). Compared to the situation a few years ago RTEMS has meanwhile gained a lot of reputation.

One thing Matthias noticed is that many users are still on older RTEMS versions. This reflects ESA having performed qualification activities on 4.8 and a couple of older versions as well as the conservative nature and long lifecycle of space projects, Hopefully with the current qualification activities, projects will begin to adopt a newer version.