Before the new set of internet regulations went into effect on March 1, a fanfiction site called Archive of Our Own (AO3) was reportedly blocked on February 29.
Muge Niu, a reporter for Quartz, writes:
I can’t begin to describe its importance to its Chinese users. It’s not mainstream like douban, but in China it‘s a refuge for literature created by and for women…
No one knows for sure but this is a widespread theory of why it was blocked: there’s a large body of fan fic works about A-list idol Xiāo Zhàn 肖战. Some fic imagines him as a woman. A lot of the fic portrays sex.