Is there a way to get Necrotic Bones aside from mob drops? They are used in the Wither Skeleton's Lootkit and I was unable to find any other way to obtain them.

Wither skulls can be dropped in the Lootkit Producer from the Wither Skeleton Lootkit as well as Necrotic Bones. The problem here is that there is no crafting recipe for necrotic bones, rendering the Lootkit unobtainable.

I think this should be reopened. Also, how to obtain Wither Skulls for building the Wither and in the end obtaining Nether Stars?

Afaik the Lootkit Producer does only produce default loot kits, which then can be crafted into mob specific lootkits.
As wither skeleton skulls are only a rare drop (when testing I took ~50 Loot kits for 3 skulls) you would need an enormous amount of necrotic bones to obtain wither skulls that way.
I don't know if the loot kits are intended to be that expensive or if I overlook sth.

Any ways I think obtaining nec. bones and wither skulls is a big issue and I don't really get why you closed the issue and think you should reopen it.

Also note that if you close a issue or PR on github you should give a reason. If you don't do so people will look at it and have no idea why it was closed which can be extremely annoying.