Weirdly, in the past (before the war update this summer) it didn't. I was in a guild that had fewer than 10 opted in, and we were unable to start a new battle until we climbed back up.
I wonder if this is a (yet another unfortunate) side effect of their new "snapshot" system? Because, essentially, the battles you are going to fight during the week are predetermined, it means that they have to include guilds that fall below the 10-person threshold. Otherwise, some guilds (mostly at the lower end, where people defect mid-week more often, but also when a top guild implodes like in your opponent's case) would not have enough eligible teams to attack.