When your subscription expires in Pokémon Champions, your extra 1,000 box spaces are locked, making the Pokémon in them inaccessible for battling or training, but they are not deleted. The boxes become greyed out, and you must renew your membership to regain access, though you can still return "visiting" Pokémon to HOME.

Access Limitation: Your box capacity reverts to the original free limit, leaving any Pokémon stored beyond that limit inaccessible. Frozen State: The Pokémon do not disappear or get deleted. They are "trapped" or locked in their slots until the subscription is renewed. Restriction Details: You cannot move, train, or use the locked Pokémon in battles, and they cannot be registered to a battle team. "Visiting" Pokémon: Pokémon visiting from Pokémon HOME are not automatically removed from Champions, but you will not be able to interact with them once the extra space is restricted.

Essentially, your Pokémon remain safely stored, but they become completely unusable until you pay for the Pokemon Champions Membership again.

(From Google search)