According to a recently a7R IV (B&H Photo Video/Adorama/Amazon) interview from Sony China by Chinese website Fengniao, Sony managers confirmed again that Sony a7S III is currently under development. Sony previous already confirmed that a7S III will beyond expectations. Sony currently has one high-end model WW942051 got registered, which means we may see a7S III announced in next 1-2 months.
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.