GNU Radio 3.9 is now available for embedded boards (including Raspberry Pis) in the BR2_EXTERNAL repository at https://github.com/oscimp/oscimp_br2_external and has been tested (on Raspberry Pi 4) with the 3.9 port of gr-acars and gr-rpitx as will be presented at FOSDEM. Of course further testing is needed to make sure all operational conditions are met until 3.9 becomes the official GNU Radio release by Buildroot.
Best, JM -- JM Friedt, FEMTO-ST Time & Frequency, 26 rue de l'Epitaphe, 25000 Besancon, France