Yup, not the first time it happened. But eventually it will go down for good I guess.
I sometimes back up my csv file from here: https://wanikanitools-golang.curiousattemptbunny.com/srs/status/history.csv?api_key=yourkeyhere
And I also have a self updating google spreadsheet (maybe based on this one? I forgot WaniKani API + Google Docs Awesomeness ) where I initially imported the data from that csv so that I had some history available.
Just getting ready for the day that my favorite WaniKani chart and script is permanently not available anymore.