Chinese website Fengniao has posted an interview with Sony managers about the recently announced Sony a7R IV (B&H Photo Video/Adorama/Amazon). During the interview, one of the Sony manager confirmed again that Sony a7S III is currently under development.
Translated from Fengniao:
Aoki Yosuke: The α7S series is also under development, and we will not disclose it in future plans. Video capture in different product lines is a feature point that is widely required, but the video features of different series are different. The α7R IV‘s pixels are very high, which brings many advantages for shooting video.
Aoki Yosuke: We are also studying the higher video specifications launched by competing products, but future plans are not convenient to share. Since the α7R IV is mainly high-definition and high-pixel, we do not particularly prefer what specifications to reach on 4K.