Why Bittrex does not support segWit address withdrawal?
Bittrex is one of the rear exchanges and services that don't accept bech32 addresses and not even withdrawing to bech32, so people should just stop using it because of that.
I think the problem is they are still using their ancient old system based only on legacy formats that can't send coins to bc1 addresses.
Bech32 bc1 addresses are universal senders, that means that you can send your coins to all other address formats.
Bittrex is not the only one and I think that Cex exchange have the same issue with not supporting bech32.
One more good source for checking bech32 support is Bitcoinops Compatibility Matrix website, but I am not sure when it was updated last time as Binance now supports bech32:
https://bitcoinops.org/en/compatibility/