I went into this assuming it would be easy to downgrade to 3.8, and discovered it was not. All of the individual 3.8.[1,2] packages are in there labeled with their version in the name, but the main 'gnuradio' package installs 3.9.
Maybe it's a good opportunity to create a gnuradio-3.8 metapackage in the PPA? If we're going to be providing long-term support for 3.8, that should probably include supporting its installation and update as well.
That said, you should be able to specifically install 3.8 and prevent it from updating by doing:
$ apt-cache policy gnuradio
$ sudo apt remove gnuradio
$ sudo apt install gnuradio=3.8.1.0~rc1-2build2
I don't know why 3.8.2 packages exist if the other installation candidate for the main package is only 3.8.1. Ideally we would have a 3.8 package which would receive updates from the maint-3.8 branch via PPA.
Nick