Pokémon games tend to change in small or significant ways as the series progresses. For Sword and Shield, one of the things that has changed is EXP sharing. If you're wondering how to turn off this feature, we've got some bad news for you.
Can you turn off EXP sharing in Pokémon Sword & Shield?
In the Sword and Shield, EXP sharing is always active and cannot be deactivated. EXP sharing divides the experience gained among all Pokemon in a party. Earlier in the series, EXP sharing was an actual item in the game, so it has been changed to a menu option. In the Sword and Shield, the menu option is no longer available and the function is always active.
The reason for this is that the developers noticed that the vast majority of players always had the feature turned on, so it changed the default setting. They felt comfortable removing the option to disable it, as long as they could still provide people with a way to level individual Pokémon, if that was their desire.
What you will need to do now if you want to train a single Pokemon is to put the rest of it in the Pokémon Center as you do so. It involves a bit of pre-planning, as you'll need to decide to be near a Pokémon Center when you decide to do so. It's never nice to lose an option in a game, especially when it comes to sheer convenience, but with no way to turn off EXP sharing in Sword & Shield, that's the only option available.