Essentially, blends continuously available in the US market since before February 15th 2007 are grandfathered in. Blends introduced into the US market date after February 15th 2007 must go through one of two Deeming processes and be FDA approved to stay available after August 8th 2021, or in some cases August 8th 2022. Blends introduced into the US market after August 8 2016 must be FDA approved PRIOR to being offered for sale in the US market.
Some blends are being introduced into the US market despite not being approved. It's a gamble that some companies are willing to take, given that they probably won't get fined until the guillotine blade drops on August 8th 2021. A great many "new" blends aren't actually new, they're just relabeled.