That limit only concerns BTC addresses (with more than 50 transactions) linked by a micropayment, linking by signing a message is still an option. Moreover, this is irrelevant as there is no limit whatsoever on Byteball addresses, so Bittrex will receive the 0.2 bytes for each byte they hold. I think I've read something about them giving the bytes to their customers though, if anyone can confirm that, it would be nice, otherwise anyone wondering should just contact their support about it.