Update 7/23/2017: BIP 91 activation completed fine with no issues. Deposits and Withdrawals turned back on.
Bitcoin Improvement Proposal 91 (BIP 91, also known as Miner Activated Soft Fork) recently locked in over 90 percent of all mining hash power, signaling majority support for this proposal. BIP 91's lock in effectively makes BIP 148 (User Activated Soft Fork scheduled for August 1) obsolete and nullifies the chances of the Bitcoin network forking through UASF.
At this time, with BIP 91 locked in, we are confident that the Bitcoin network will successfully activate and enforce BIP 91 blocks. Bittrex will be monitoring this situation closely. We do not anticipate any issues with BIP 91 activation. However, Bittrex will close deposits and withdrawals of Bitcoin (including 2nd layer protocol XCP/OMNI and associated assets) 4 hours prior to the BIP 91 activation block 477,120 anticipated to be at July 23, 12am PST (7am UTC). The wallet will remain closed until the majority hash power chain is clearly established. Trading will remain active during this time. In the event of a chain split, Bittrex will take all reasonable steps to ensure that customer funds can be preserved on both chains.