I noticed the Bittrex wallet maintenance as well since Nov 28.  Their FAQs suggest blockchain upgrades are one of the main reasons this happens. https://bittrex.zendesk.com/hc/en-us/articles/115000233911-What-does-it-mean-when-a-wallet-is-in-maintenance-  They will need to upgrade their xem wallet, so let's hope they get around to it soon.  Usually wallets will be frozen like this for a while.  I just hope they are preparing to announce opt in support!