As I couldn't find this information on the internet and had to figure it out myself I'm posting this info here for search engines to be able to pick it up:
If you run flightradar24 on a regular Debian VM as I do you might have asked yourself what the correct /etc/apt/sources.list
entry is. And the answer seems to be a rather strange one:
# flightradar24
deb [arch=amd64] http://repo.feed.flightradar24.com flightradar24 raspberrypi-stable
After adding this repo you should now be able to install flightradar24 with:
sudo apt install fr24feed
and get regular updates via apt.
Happy ADSB tracking 😊