http://nijibox.ohflip.com/ke/
Yes, Nijikaku's servers are dead and there are no more updates. Aside from other weird fighting games there's isn't anything like it besides the meme Mugen characters.
I wanted to make source accurate KSS versions of the Magical Sweeper enemies, so here they are: https://drive.google.com/file/d/1VzTsFFFcPp_JBZsk71XZpICtX3hOYCyf/view?usp=sharing