The closest we’ve ever gotten was when she pushed him down after returning home while it was raining, put her hand on his cheek, and called him “good boy” in chapter 23.
This is not about some sick sorts of bdsm, but I can totally see Shikimori using a dildo on Izumi and how they are both fine with it.
Shikimori: I love you, Izumi-san
Izumi is like: "Gods, my girlfriend/wife is so cool"