The game is essentially complete. The dev had plans to add additional content, for example more content with Vendetta (the white haired girl that stays in the hub area and appears in 1 or 2 game overs), but then the fiasco with Unity devs happened (when they decided that it would be cool to force game devs pay them money for selling games made with Unity, including retroactively), and when Unity devs backed down, the newer Unity versions broke Night of Revenge's animations (or something along those lines, bottom line is that newer unity version+Night of Revenge=broken Night of Revenge), so the last thing he said about that is that "it would take a lot of effort to update Night of Revenge to a newer version of Unity and make additional content to it".
Plus he's been working on a new game, which I haven't tried to find more about, but it's probably higher a higher priority for D-Lis than NoR, especially considering that it's been a few years since the last Night of Revenge update.
EDIT:
Actually, wait a second,
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and while it's unfortunately members-only, there's a suspiciously well liked comment that, when translated, says " Yeahhh DLC for NoR, i like it and hope you will update soon ", so maybe he still plans to add more content to NoR?