I think it's staying vacant? I can't really find out through the depressing posts at S48.
Here's a couple of radio links from Nihongog
(Sayaka's message)
The links after Sayaka's message are of random members coming into the studio to show support. We get Sae, Takamina, Yuka, Nacchi, Natsumi (Matsubara), Kana, and Umeda (along with the radio hosts Tomo~mi and Mocchi)
I think it's really effed up for her to step down, but it's admirable that she wants to earn her spot again.