All trading for US Poloniex customers has ended (effective as of November 1, 2019) and US Poloniex customers should stop depositing immediately and withdraw their assets through the new Poloniex US Withdrawal site poloniexus.com as soon as possible. These changes are a result of Poloniex spinning out from Circle into a new company that is exclusively focused on the international market and no longer serving US customers.
While US Poloniex customers should withdraw their assets as soon as possible, they will continue to have access to all crypto assets through Circle until at least December 15, 2019.
For further information on this, please reference this Blog post.