Underage players (those under 21) do not have to be registered in the 25-player squad list for English Football League (EFL) clubs.
EFL Squad Registration Rules
25-Player Limit: Clubs must submit a list of up to 25 players for league matches.
U21 Exemption: Players under the age of 21 (as of January 1 of the relevant season) do not count toward this limit and do not need to be registered in the 25-player squad.
Homegrown & Academy Players: Clubs are encouraged to develop young players, and U21s from academies can freely participate without impacting the squad limit.
So, underage players can still play in league matches without being officially listed in the 25-player squad.