What prevents us from adding
parcimonie to Tails?
The author is a nice guy :)
It doesn't. I install gnome-shell-pomodoro and have to enable it manually each time and then restart GNOME Shell.
FTR this is another, additional/different problem than the one I was referring to: even if
gnome-shell-pomodoro was installed (but disabled) by default, you would have to enable it and restart GNOME Shell anyway. While parcimonie installed via ASP — once it has a GNOME Shell extension — would be affected by two problems: 1. the parcimonie daemon would not auto-start; 2. the user would need to enable the GNOME Shell extension + restart GNOME Shell.
And in passing, I think that currently on Wayland there's no way to restart GNOME Shell without restarting the full GNOME session, which we don't support in Tails. If that's a blocker for the move to Wayland in your opinion, or you feel the need to discuss this meta aspect, please file a subtask of #12213 :)