This is now status-completed
It appears that moderators can no longer decline flags with a custom reason. When we go to decline a flag and choose the "other" option, no text box appears and we can't enter a reason:
I observed this on Safari 10.0 running on macOS Sierra (10.12), and another moderator reproduced it in Chrome. They stated that forcibly running
$("#dis_Other").toggle()
will bring back the text box.