The magnetometers and charged particle detectors are still operating. So they are measuring the galactic magnetic field, and cosmic rays and the gas in interstellar space. The results are more or less what was predicted, though the exact boundary of the sun's influence was only discovered when Voyager 1 and 2 crossed over it in 2012 and 2018 respectively. Beyond that, yes, they're basically assuring us the sun is still there and that space is very empty. I don't think anyone expects the interstellar gas to vary in density on the timeframe that the Voyagers will be able to observe it but, I guess we'll find out!

Edit: I spoke way too soon. It varies, as discovered with Voyager measurements recently: https://www.jpl.nasa.gov/news/as-nasas-voyager-1-surveys-int...