I got an email from the exchange I use stating the Istanbul hard fork is to occur at block 9069000 at approximately 15:44:36 PST on December 7. It happened already but they have suspended all deposits and withdrawals from happening until further notice.
It is not a contentious fork and so does not result in any new coins from it happening.
So why would they decide to suspend all transactions of ethereum?
This is an upgrade for ethereum network, and change consensus algorithm, so I think they suspended deposit and withdrawn just to make sure nothing is wrong with the ethereum transaction before the upgrade is complete and the system works stably.