The 22-year-old trained regularly with the first team, but never made a senior appearance during his time with the club.
Plain joined Cherries in 2018, spending two years with the under-18s, before moving up to the development squad.
He played his youth football for Winchester City and AFC Bitterne Jaguars.
Part of the squad which reached the FA Youth Cup quarter-finals in 2019, Plain gained experience away on loan in non-league with the likes of Ringwood Town, Bashley and Wimborne Town.