I find it weird though that Bittrex doesn't allow funds to be recovered after their stated withdrawal time frame. It's clear that not everyone will be able to withdraw their funds on time.

There should be a policy in place that makes sure users who didn't manage to withdraw their funds will still be able to get them. They state assets become unrecoverable, but how exactly? Do they burn them or what?

It's way more likely that the unclaimed funds become property of Bittrex and they end up selling them elsewhere, could be to recover some of the funds they lost during the attack they suffered from.

OP, can you send them an email explaining your situation without threatening to report them or follow up with legal action? Not much to lose anyway.