Indeed, derby is the team part of the game Hay Day offers. Just like Clan War in Clash of Clans.
The day opt in/out was added to Clash as a leader ability the community was happier. No longer did leaders have to boot players who were inactive for a brief period of time and unable to opt themselves out.
In clan wars, an inactive member is just as much a liability as an inactive member in a derby. Opting them out is far nicer than booting them out of the group completely.

