This is mostly happening for the skeleton's rib bone and creeper catalyzing gland. How rarely do these things drop?

Using Forge 47.2.0 for 1.20.1, with mod version 2.0.36.1199

Yes they require player kill

They are pretty rare if you don't have any looting or severing on your weapon, but they definitely drop. In that case the average drop rate is 1 in 50 skeletons or creepers killed. you really want to be using severing / looting if you want higher chance.

Thanks.

Also, do the drops require the mob to be killed by a player?