I don't think Poloniex ever shut down registrations like Binance or Bittrex did. They did institute a pretty invasive KYC process for new customers last year, and they made KYC mandatory this year, but they never shut down registrations. That said, I don't know how long it takes to get verified before you can start trading.
Poloniex has been batching transactions for years, and they've always offered significantly lower fees than virtually all other exchanges, so this isn't a big deal to me. When you get down to it, the real issue for Coinbase wasn't Segwit, but inefficient wallet infrastructure and no batching.