Kostiuk next competed at the European Youth Olympic Festival in Baku, Azerbaijan, again alongside Chepurnyi and Kovtun. They won gold as a team and individually Kostiuk won silver on parallel bars and bronze on pommel horse.
Kostiuk competed at the 2020 European Championships held in Mersin, Turkey. While there he helped the Ukrainian junior team win gold.[4][5] Individually he won silver in the all-around behind compatriot Illia Kovtun and won silver on rings and bronze on horizontal bar.[5][6][7]
Senior
2021
Kostiuk turned senior in 2021 and made his senior international debut at the 2021 European Championships; he finished seventh in the all-around.[8]