The original timebows from six years ago abused the extremely powerful 2-turn sundial, which was nerfed to one turn. Following the nerf, a rainbow called CCYB took hold, it's defenses centered around Shard of Gratitude, which back then healed for 5 and costed 2 neutral quanta. Then, Shard of Gratitude was nerfed, but Shard of Readiness was buffed, allowing for a deck of unprecedented grinding power called Instosis. When Shard of Readiness was nerfed, Instosis became unplayable.
So, it was primarily the changes to Shards that brought down the old rainbow FG grinders, but they've also opened up some new one, like Brave Malignant Ball.