for me forgotten mainly translates to "never ever been viable in any tier ever" so true champion pokemon like maractus and vanilluxe are definitely good examples imo. and then there's also this mon who's so awful both competitively and design wise that it hasn't even brought up here yet: unfezant :(
Following @ZoroDark's logic, Mothim and Wigglytuff certainly fit the bill. With both of them, it's simply a shrug and a "who cares" treatment. Mothim doesn't really do much of anything and Wigglytuff is kind of an inferior Mega Audino at this point, and we all know how badly that thing sucks in terms of other Megas.