This is pixiv Secretariat.

The posting option "Delete in 24 hours", that we offered as an experimental function starting from April 19th 2018, will be made unavailable on July 3rd 2018, 12:00 JST.

What's the "Delete in 24 hours" posting option?
By setting this function when posting a work, your post will be automatically deleted after 24 hours.
You can use this function when you want to publish your work for a limited time, or when posting your production process.

Thank you for your cooperation regarding the experimentation of this new function.
We will use the data we collected to further improve our service.

Thank you for your patronage with pixiv.