Dual citizenship is technically illegal in Ukraine but is widely tolerated in practice.

There were lots of stories in the explicitly Jewish/Israeli media about Jews being evacuated to Israel right before and during the initial invasion, and I'm pretty sure none of them were forced to permanently give up Ukrainian citizenship. I recall reading a couple stories about Zelensky's parents moving to Israel and living in some multi-million dollar mansion complete with servants and their own security detail.

I doubt any of these oligarchs will have to give one penny they looted back to Ukraine and in fact are still probably profiting from the various Ukrainian industries and government services they monopolized. The only thing thats changed is their physical location, and some of them probably already had part time residences in Israel, among other places. This supposed gesture is likely just window dressing that means nothing in practice aside from shielding them from investigation or scrutiny now that the world is watching Ukraine, not that the Jewish dominated media and western bureacracy wasnt already covering for them.