it's 100% not YF.
Firstly, YF is exceptionally rare. Secondly, it doesn't look anything like that
Yellow fungus will have an actual texture to it most times, like an actual growth.
As they get older and sheds take longer and longer, the sheds tend to stick around.That causes them to darken a bit, and pick up on stains and dirt.
If you want some more reassurance, search this site for "Yellow fungus". You'll find 100 threads of people posting the same basic image you are, and maybe 1 or 2 of them are actually yellow fungus. All others are just old/dirty/stained shed.
-Brandon