Proof is not hard to look for - it is everywhere. Every social channel is flooded with comments of people who have been scammed by Bittrex. Funds worth in (many) millions $ US are seized by Bittrex.
I also don't understand what they are doing on twitter, picking a few users and solving their tickets, are they teasing people or what is going on? It's also funny that people who are verified don't give a fuck about what's going on and they continue to use the exchange...