Nonsense. An organsation cannot lawfully have a contract that places itself beyond the law. The only barrier is a practical one: whether you have the means and resources to pursue a legal action.
But I think the first step should be a well written letter to the Chief Executive.
Poloniex has delisted Nushares. Therefore, Poloniex does not wish to have further involvement (for now). Therefore, it has a natural duty to conclude its affairs in a proper means - for instance, by returning to customers what is theirs. To terminate activity without tying up loose ends goes against all business norms. While a legal action is perfectly possible, it should not be necessary. Acknowlege that now is a hectic time in crypto, due to the volatility, but ask that the chief executive finds time to ensure that Poloniex fulfils its obligations to former customers.
Nushares has an active community and road map, so it is in everybody’s
interest that matters are resolved, in case Nushares becomes prominent in future.
Provide a list of ticket numbers, etc.
And I stress - write an actual letter. Find the chief exec’s name and the postal address.