In 21w13a, going under y=0 removes the gradient effect from the fog and leaves only 100% or 0% fog. This also affects the nether.
MC-206610 At certain camera angles around -25.4 in second person, the lava fog effect applies when it shouldn't
MC-221680 The nether fog behaves rarely when going deeper than y=0
MC-221672 The end fog stops working when higher than y=340