Last season, as part of the Mets’ annual hazing ritual, rookies were made to wear uniforms from the classic flick “A League of Their Own,” based on the creation of a women’s professional baseball league during World War II. Then, they were sent out to retrieve coffee and doughnuts for the team’s veteran players.

“For me, it was a good team bonding experience,” said outfielder Brandon Nimmo, among the Mets rookies to don a yellow skirt as a member of the Racine Belles (while others were Rockford Peaches). “But I was also very lucky to have a group of veterans that didn’t exploit us or anything. They kept it fun . . . so I really enjoyed it.”

But the Mets, and every other team in baseball, will no longer be permitted to dress up as women under a new provision in baseball’s collective bargaining agreement, one that spells out restrictions in hazing rituals that have become commonplace in clubhouses.

According to The Associated Press, the rules keep players from “wearing costumes that may be offensive to individuals based on their race, sex, nationality, age, sexual orientation, gender identity or other characteristic.”

The new policy also includes anti-bullying language though it stops short of outlawing all initiation rituals, which in recent years have become social media staples near the end of the regular season, when such dress-up days are common.

“It’s something I’m very concerned about as a potential issue,” said Mets general manager Sandy Alderson, the former Marine who supports the new rules. “I’ve seen it in the military. For all the camaraderie it’s supposed to promote, it’s divisive and I think undercuts morale. So you’ve got to be very careful about that.”

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But players often view initiation as a team-building ritual that promotes camaraderie.

On Tuesday, Nimmo was among those to lament the new policy. He was at Citi Field, dressed as an elf alongside Jose Reyes and Noah Syndergaard, who arrived in a blue Santa Claus suit as part of the team’s annual Kids Holiday Party.

“It’s just kind of the way things are when you get in,” Nimmo said. “You have to put in your time. It’s just like any job. I enjoyed it but like I said, I only have one experience with our veterans and I know that we’re really lucky to have a great group of veteran guys. I guess I’m sad to see that go but obviously it’s not my decision . . . I’m glad that I got to partake in it last year. I wouldn’t trade that.”

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Syndergaard was in the rookie class of 2015, which got dressed up in superhero-themed costumes that included revealing short-shorts. That season, the rookies spent a night on the town in Cincinnati dressed in full costume.

“It was a fun time, it’s part of it, you’ve got to embrace it,” Syndergaard said. “I wouldn’t really say I understand it, but I’ve got no say in it so I’m not going to be bitter about it.”

Still, Alderson questioned the benefits of the practice.

“Is it constructive? Is it useful? Is it juvenile? It’s probably juvenile,” he said. “It’s probably not useful or constructive in too many ways.”

Notes & quotes: Syndergaard did not back off his Twitter declaration last week that the Mets have the best rotation in baseball. “Their ability is outstanding,” he said. “The fact that we’re all a tight-knit brotherhood within ourselves, we’re always rooting for one another. We’ve just got a lot of good camaraderie.” . . . The Mets still hope to trade outfielder Jay Bruce, though little has changed on that front since last week’s winter meetings. “Pretty much status quo,” Alderson said of an outfield market that has developed slowly . . . Third baseman David Wright is simulating baseball activity in California, where he continues to work with back and neck specialists. He’s expected to be at the team’s complex in Florida in early to mid-January . . . Reyes said he’s prepared to play in the outfield as the Mets introduce him to a utility role next season . . . Major League Baseball’s charity auction to honor late Mets PR exec Shannon Forde raised $235,000, which will go toward renovating a field in her hometown of Little Ferry, New Jersey, that will be named for her.