He's popped up here and there. Here he is on the main art for Luca's Overdose cover, which came out on December 8th.It also lines up with something I said IDFK time back. Ninjisanji is trying to memoryhole MYSTA not Nina who has appeared several times after her graduation (most notably the backround of reimus splashscreen) but Mysta is nowhere to be seen.
And if you want to believe people's rrats, Vox alluded to him during their 2nd anniversary game show by talking about tax evasion lol.