I recently upgraded to Ox 0.4.0 with no issues. It’s nice to remove the need for a plugin. Out of curiosity, though, I was wondering what drove the decision to remove IO? Thanks in advance! Cheers! 
There’s some background here: x.com.
Summarizing the thread, the only role that the IO capability played was that of tracking of I/O operations. It didn’t “do” anything: it isn’t required to implement any functionality. And people already usually know if a function performs I/O or not, so that information isn’t that valuable.
Adam, thanks for the inside scoop!