Only if he's under 21 on loan from another club, signed players under 21 don't count towards the 25 man squad
Jack Sims was not part of the 25 man squad, he was named on the bench as a
club developed under 21 which every
club have to do.
Taken from the EFL Rules
Eight of the 25 must be home-grown - a player, regardless of nationality, that has been registered with any club associated with the English or Welsh FA for at least three years before reaching 12 years old.
Any player aged 21 or under (
on the books) doesn’t have to be included in the 25-man squad.
However, unlike League One, any loan player - irrespective of age - must be included.
Ps. I'm not being Arsey and your one of the posters i enjoy