Poloniex claimed they needed STEEM development to address something before they could enable STEEM wallets.
This is just a generic error message they put on any disabled market so that they don't get so many support tickets.
I can't say for sure if Poloniex needed the witnesses to have a hardfork-supermajority upgrade to 19.2, but my guess is they probably didn't notice or care and it was a coincidence.
You are correct. But just to verify, I went in to Poloniex to see if there was another disabled symbol.
Yup. Generic message.