This has returned in 5.4.0b16. I need to see how far it regresses still. Right now, exhibited behavior is this:

1) Launch a project in Development Mode and select "Script Debugging"
2) Open a script in that project and put some breakpoints in
3) Select "Attach to Process"
4) The device is not recognized

UPDATE:
I'm also unable to get this working on 5.5.0b4. Could only test the emulator with Win 8.1 as my device isn't recoginzed by the IDE and OSX won't deploy at all.

Repo steps are the same as above.