Release notes: https://platform.stability.ai/docs/release-notes#stable-image-v1-release

Overview

We’re excited to announce the release of the Stable Diffusion v1.6 engine to the REST API! This model is designed to be a higher quality, more cost-effective alternative to stable-diffusion-v1-5 and is ideal for users who are looking to replace it in their workflows. stable-diffusion-v1-6 supports aspect ratios in 64px increments from 320px to 1536px on either side, and has been optimized to provide higher quality 512px generations when compared to stable-diffusion-v1-5.

They’re also deprecating SD1.5 on 11/15/2023.