Version 1.16.100 changed biome fog to be data-driven, via JSON files in the "fogs" folder at the root of a resource pack, as explained in this article. As is documented in MC-68535, all biomes in the Nether have fog at a fixed distance of about 96 blocks. However, instead of being represented via a fog with a "render_distance_type" of "fixed", the Nether fog remains hardcoded, making it impossible to modify this fog via resource packs or the "/fog" command.
Since the Nether fog remains fixed regardless of render distance, it presumably exists for aesthetic and not performance reasons, and thus should use the same method of definition as all the other types of aesthetic fog in Bedrock Edition.
Still affecting 1.16.210