As a note for anyone sticking to Arduino 1.8.19 after installing 4.4.0 experimental:
If you have both official and experimental versions installed simultaneously, the 1.8.19 version of the Tools>Board menu shows both plugin packages as just “Proffieboard”.
The newer version 2.x Arduino is kind enough to show a difference as “Proffieboard” and “Proffieboard Experimental”, which makes knowing which you’re choosing easy.
To mimic this labeling when using Arduino 1.8.19, you can edit the text file found at
Arduino15/packages/proffieboard_experimental/hardware/stm32l4/4.0.0/platform.txt
The “name” line can be whatever you want to show up in the Tools>Board menu.
I made mine like this:
name=Proffieboard_experimental