Issue #13618 has been updated by mame (Yusuke Endoh).
I comment in compliance with hsbt's request.
Basically I agree with ko1; Thread is considered harmful.  Casual Rubyists (including I) had better not use it.
However, I'm not against introducing the feature in question as a professional feature for mature Rubyists.
> A *fiber* is a unit of execution that must be manually scheduled by the application.
I believe that this feature should be introduced with another name.  I have no counterproposal, though.  Sorry.