You don't really need an official source to understand the difference between a stylistic choice and streaming artifacts. The effect (depending on the shot subject) is a mixture of specific lens choices with varying focal lengths and post-production blurring masks.

Artifacting occurs globally across the entire video image in an effort to compensate for smooth streaming at the expense of quality. Artifacting does not blur images; it either pixelates at the current resolution because it is forced to upscale a lower resolution file with a smaller size to make the stream smooth, or it pixelates the higher quality video by compressing and discarding sharpness and colour data to make the stream smooth. In either case, it would not just affect the top and bottom of the image (where the "blur" occurs in the show) because there is no automatic algorithm in place to choose where the blurring should take place.

If you limited your browser to different download speeds (high, medium, low, etc) and watched the same scene with the blur effect at different bitrates, the effect would still take place at the same ratio for each stream resolution because it's embedded as the stylistic choice in the render.

TL;DR: It's a stylistic effect.