So as of October 10th, 4 pm pst i got my shiny wither skelly head, the only head needed to finish my collection, i got 3 shiny heads in 4 days, sad thing is it took me 4 days to get 3 shiny heads and spent like 3 months without a single one xD Screenshots below, to prove its actually on autcraft i have the chat shown in one of them
Shiny heads are a very rare type of loot. There are also non-shiny heads which are a bit easier to get. If you use the mob farms for a long time, you might notice that you get a normal head in your loot after a while. Shiny ones are a lot more rate. I only have one so far.