The 19-year-old right-back, a graduate of the Hale End academy who joined Arsenal's youth system at the age of seven, published a farewell message on Instagram that laid out what the club means to him in personal rather than professional terms.
"It's hard to put into words what this club means to me," Nichols wrote.
"This hasn't just been a team I've played for. It's been a second family, a place where I started as a little boy, made lifelong memories, and shared unforgettable moments both on and off the pitch with my brothers. Over the past decade I've experienced the highs, the lows, the victories, the disappointments, and everything in between.
"Through it all, my love for this club has never changed. Wearing this badge has always been an honour, and every time I stepped onto the pitch I gave everything I had because this club means so much to me. I want to thank every team-mate, coach, supporter, and most importantly my family for making these years so special. I loved every single moment. Once a Gunner always a Gunner."
Nichols made one competitive appearance for the senior side — as a substitute in a Carabao Cup tie against Bolton Wanderers in September 2024. He trained with Mikel Arteta's first-team squad throughout the 2025-26 season without adding a further senior appearance.
NK Kustosija, based in Zagreb, have built a reputation for acquiring players from the academies of Premier League clubs and developing them for onward moves to the Bundesliga and La Liga. Fabrizio Romano confirmed the details of the agreement.