I’ve had a quick look through the ToS and it seems that clause 8.3 is relevant
8.3. Support for Tokens
Bittrex retains the right, in its sole discretion, to determine whether to support transfer, storage or trading of any Token using the Services, and may discontinue or terminate any support for any Token at any time for any or no reason. Unless otherwise required by law or law enforcement, Bittrex will make reasonable efforts to notify you of its decision to cease to support of a Token. If Bittrex ceases to support transfer or storage of a particular Token using the Services, Bittrex will use commercially reasonable efforts to notify you at least 14 days prior so as to afford you with an opportunity to transfer the affected Token from your Hosted Wallet to an External Address. If you do not transfer the affected Token out of your Hosted Wallet prior to cessation of support for the Token by Bittrex, the Token may be lost due to your inability to access, transfer or otherwise control the Token. Bittrex will not be liable to you for any losses, liability or expenses related to its decision to cease any support for any Token.
This wording seems less aggressive than the message in the link to their FAQ provided by @JPL above.
Clause 8.8 may also be relevant here:
8.8. Unclaimed Property
If your Bittrex Account has been inactive and you have not responded to reasonable attempts by Bittrex to contact you for a period of several years (as defined by the relevant state statutes), Bittrex may have an obligation to report any Tokens in your Hosted Wallet to the applicable governmental entity as unclaimed property. If this happens, Bittrex will attempt to contact you using the contact information provided by you. If you do not respond, Bittrex may be obligated to turn over any Tokens in your Hosted Wallet to the applicable governmental entity after deducting any fees payable to Bittrex.
If there has been known instances of Bittrex refusing to provide MAID to the owners of their wallets then I would see than as theft and a very serious matter. Is there sufficient evidence of that happening here?