At the time of Stoneforge Mystics’ unbanning in 2019, many people were wary and believed the creature would turn out to be too strong for Modern.
Since then, Stoneforge Mystic has seen play in multiple different archetypes – from UW Control as a quick way to close a game, to one of the most aggressive decks of the format, Hammer Time.
One of the arguments against unbanning Stoneforge Mystic at the time was that it will benefit from any equipments printed in future sets. The idea behind that argument was that Stoneforge Mystic would either restrict the design of any new equipments, or eventually be banned again if more powerful equipments were to be released.
As it turns out, the power level of Stoneforge Mystic is just right for Modern. Since its unbanning, new strong equipments (Kaldra Compleat, The Reality Chip, Colossus Hammer) have been printed and have found a home in a very solid Tier 1 deck (Hammer Time) alongside Stoneforge Mystic, without being too powerful for Modern.
As of now, the remaining Stoneforge Mystics have mostly disappeared, but this can easily change in the future as well.