Why has The List suddenly become irrelevant in season 2? Those who have failed the city, whom Oliver hadn't crossed off by the end of season 1, are still out there. Just because their plan of destroying the Glades that they were working towards actually succeeded, doesn't now mean that they shouldn't be punished. It probably means that now they are even more deserving of a visit from the hooded man.
Merlyn's Undertaking, for which they all had some part in, led to the death of thousands of people, and except for Moira Queen, no one else has been held up as responsible. Not by the law, nor by Oliver and team who are well aware of the names on The List.
The List is now even more relevant; if Oliver couldn't stop them before the fact, then they are even more deserving of punishment after the fact, because they have gotten away with it.
At the very least Oliver should turn The List over to the police with the message, "These people are responsible for destroying the Glades."