The matchmaking for Team Wars is based only on each team’s amount of Trophies. Trophies are an accurate representation of each team’s performance in Wars and, therefore, they provide a better-quality matchmaking.

 
Matchmaking in Legendary Leagues
Legendary Leagues have a fixed matchmaking. This means that matchmaking will happen at certain times and all teams will be matched simultaneously, without any Leads or Co-Leads having to request a matchmaking. The system will assume that your Team wants to go into war unless it opts out. Breaks between wars will last 8 hours.
 
Matchmaking OPT OUT for Legendary Leagues only
During this 8-hour break, Team Leads and Co-leads have the chance to opt out of the matchmaking, thus skipping the war. The whole team has to wait for another matchmaking to start automatically (56 hours later). There is no penalty for opting out. Your Team just won’t get rewards, which is usual for not going to war. Your team can also opt out as many times as you want, there is no limit.

Matchmaking criteria for Legendary Leagues only

Rival repetition
You can’t be matched against the same team in 8 consecutive wars or more. This means that your team will match only with teams that are 10 ranks above or below and you will face the same team maximum 3 times in a season.

Team size
At least 25 team members are needed to compete. If you have less than that you won’t be included in the next matchmaking and your Team won’t go to war. It will count as the same as opting out.