According to a story out today by the ATU's Rick Karlin, 1.6 million named were purged from the voter rolls. To put that in perspective that's more than 10% of total registered voters (using the state Board of Election enrollment figures for March.)
The issue with purges, as detailed in a recent Brennan Center report, is that they're usually done in secret. As the report details tens, if not hundreds, of thousands of voters are wrongly purged around the country; that's scary.
There may be legal issues with New York's purge under the federal National Voter Registration Act and state election law. Basically, systematic purges cannot occur within 90 days on an election. (Tell that to Colorado's Secretary of State who is using the depends-on-your-definition-of-systematic defense for their state's purge that happened within the 90-day period.) Moreover, possible duplicate names shouldn't be removed from the rolls until they are moved to an inactive list and after two federal elections pass.
New York's purge may be entirely lawful, but given the large numbers involved the Board of Elections should immediately explain how and when those names were purged.