In Gen6, Swagger was used in a playstyle called SwagPlay where a Pokemon with the Prankster ability used Swagger and Thunder Wave to make it highly unlikely for the opponent to attack, followed by Foul Play with does more damaged after the opponent's attack is boosted by Swagger. The attack boost from Swagger even makes the opponent do more damage to themselves when they hit themselves in confusion.
So, with just 1 priority move, your opponent has a 50% chance to hurt themselves with a boosted attack, skipping their turn. Then you would either be able to paralyse them, adding more luck to whether they can use a move or not, as well as lowering their speed. Then using Foul Play to do massive damage due to the stat boost.
Pokemon commonly used for this strategy were Klefki and Liepard. When discussed on competitive forums, the outright banning of those Pokemon was considered, or banning Swagger when used by a Pokemon with Prankster. In the end, it was decided to ban Swagger instead as those Pokemon had other uses.
tl;dr: Swagger is a luck based move with no real competitive use to it. It was used as a cheese strategy.
Despite the Gen7 changes to Swagger, Confusion and Thunder Wave, the move is still considered to be too luck based with no competitive skill involved. Thus, it remains banned. It's unlikely that Swagger will ever be unbanned.