From what i have heard from people is that it's not bannable if it was retrieved from the nether by beheading a wither skeleton, as this is not glitching, it's not bannable. So my answer is Yes and No. If the skull is renamed, then I suggest give it in to a mod.
You where lied too. There was no wither skeleton killed, all the ones in game came after Tanners /staff plugin. They're all glitched items.
They will be arrested for possession of an illegal substance. Making, selling, buying, or using illegal drugs will result in charges. Most likely jail time. The same goes for illegal (glitched in) items here on Mineverse. Though how the punishment is carried out varies from real life to Mineverse.
It doesn't matter if the glitched item wasn't glitched in by you, you are in possession of something not allowed.