When they remove credible developer projects then it's obviously for other reasons. There is no minimum volume requirement if the project is still solid.
https://bittrex.zendesk.com/hc/en-us/articles/360022158031-Why-Bittrex-Removes-Certain-Coins-or-Tokens
Bittrex has delisted (lot from Bittrex US) many coins just based on regulation reasons (unable to prove that coin is not security), rest are probably delisted because they are broken or developers have abandoned it officially or by not maintaining it anymore.
Obyte wallet (deposit/withdraw) on Bittrex goes automatic maintenance mode every time when nobody (including witnesses) post for long time. Every cryptocurrency project leader is in communication with Bittrex and these issues have been solved fast. Last time when Obyte network had bigger issues was this summer (nodes had issues couple hours and last summer (nodes were down 2 days).
Since Obyte is one of the few innovative projects using DAG, it's not security and it actually works and gets improved all the time - there is no reason to delist it for Bittrex.
Obyte is also not just maintained, but it's still improved with lot of new tech. For example, there is Autonomous Agents being tested on testnet and developer contest on it just ended.
Still holding a lot and was still accumulating (until cryptopia took a few hundred GB worth away) so obviously not saying it was dishonest or corrupt. Glad to see it had not been abandoned.
Doesn't make no sense to hold hundreds of GB on centralized exchanges. 100GB might not be worth much at the moment, but it seems nuts to keep that much on exchange.