Being a completely new user, I too was very surprised that the “activate” button doesn’t also “deactivate” – especially since Steinberg’s Cubase uses a similar button that does deactivate.
There are a few different functional uses of such a deactivate button – for me, a very simple one is to be able to turn off all vst instruments in an open project so that I can use my midi keyboard for other tasks (eg, reading thru a student’s work/ analysing a score / giving myself a break by improvising at the keyboard).
One might be able to create a script/macro that opens Playback Template and changes to/from Silent playback, but there’s another way that I stumbled upon: In Write mode, if no event is selected on any staff, Alt+S will solo the “Dorico Beep” (metronome sound). If one isn’t using the metronome (or has assigned a different sound to it than the DB?), soloing the Dorico Beep effectively mutes all staves. (Shift+Alt+S removes the solo and unmutes everone.)
Still, if I’m still using Dorico by then, I will definitely applaud the code that makes that Activate button toggle with Deactivate. When it comes to workarounds, what would be “just another damn workaround (sigh. . .)” in Finale stands out far more in Dorico, simply because Dorico is otherwise a gorgeously elegant program.