I'm reporting this on behalf of a friend (I'll call him Bob hereon) who's not registered on this forum. I know him personally and trust him 100%, he also gave evidence (screens, mail exchange with support, etc) of all of his statements.
1) Bob opened his account on Bittrex in 2016 when verification was NOT mandatory according to their TOS;
2) he's been using the exchange seldomly and made just a few trades for a small amount of altcoins (no fiat, nor bitcoins);
3) in 2018 Bittrex covertly disabled Bob's account without any notice and for absolutely no reason;
4) when Bob found out his account was disabled, he contacted the support and asked for some explanation. They only answered that in order to enable his account he had to go through KYC and send them his documents;
5) shortly thereafter, Bittrex changed the TOS and introduced mandatory KYC;
6) Bob, who hadn't actually used the exchange since the old TOS still applied, claimed his right to reject the change and close his account, and asked to withdraw all of his coins;
7) Bittrex denied him to close the account and withdraw his funds without prior verification.
This is clearly a scam and an attempt to illegally hold Bob's funds since:
1) Bob chose to sign up with Bittrex precisely because they were not requiring KYC. He would never have registered if he'd known verification was mandatory;
2) if Bittrex had some valid reason to disable Bob's account (they never had, actually), then Bob would still have the right to be promptly informed about the block and the reason why. They acted instead in a shady way and with total lack of transparency;
3) whenever some service provider unilaterally changes the TOS, a customer has always the right to reject the change and close his account. On account's termination, the old TOS still apply, that means Bob has still the right to close his account and withdraw his funds without going through verification.
I've seen a number of other scam accusations on this shady exchange, let alone allegations of fraud, deception and market-manipulation:
https://www.coindesk.com/bittrex-poloniex-sued-tether-bitcoin-manipulation, then I think Bittrex deserves all scammed users join and start a lawsuit to make those scammers pay for their crimes once and for all.