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The Tensor G3 is the first smartphone SoC to support hardware-accelerated AV1 encoding. It supports AV1 encoding at up to 4K60. No application (including the Pixel Camera app) takes advantage of this, though, likely due to a lack of platform support.
Because there is no need for now compared to cost and efficiency.
Decoding - yes and I believe the last gen has that because streaming services are switching to AV1. However, only creators really would stream and make use of the encoding features.