You can still do it, you just have to jump through some more hoops. You have to be able to if you think about it, otherwise you couldn't develop an extension
Edit: having done a bit of reading, the above may be incorrect now
Well if I remember correctly, they make it really annoying. Like you can only have one developer extension at a time, and you need to set it up again every time you restart chrome. I doubt most people will bother.
Just did some testing. Chrome stable 54.0.2840.59 on linux. Put on OP's extension from source and an extension of my own that was already in there. No issues. Then I packed both of them and installed them both packed, again no issues.
You're not crazy though, the internet seems to agree that there was an issue.
