My project is Single-Threaded Reactor
It’s based on 0.15.2, just plain std.posix
All my design is based on assumption that io is on single thread.
How I can achieve this model in 0.16 (with or without Io)?
The non-blocking implementations Evented are only proof-of-concept quality rn, very WIP and I don’t think any non-blocking implementations support networking yet.
There is zio which provides a non-blocking implementation, however its 0.16 branch is 2 months without work, so it is almost certainly not compatible with the release.
You’ll have to do your own thing, which you could make an Io implementation if you want, but that may add an unreasonable amount of work on top of what you need. It’s up to you.
so in short form an answer
No - you can not (with or without Io) 
With Io: Currently no, in the future (pun intended) yes
without Io: depends, std may implement the api’s you need. Or it may not. regardless they will be lower level and platform specific.
i look at the Io implementation and remember C++ frameworks
looks as binary solution or Io or …
still i don’t understand the reason of tightly coupling api wrappers with Io framework