When I "save as" a file for which I've set readonly flag, after save it remains readonly. I don't see a sense in this. The readonly flag was to protect the original file (which may be mission critical and should not change), but I see no reason for the newly saved file to remain readonly. Reproducible: Always
I tested this by setting a file to be read only for owner. When I saved it under a new name, the new file had also write access for owner. Arch Linux 64-bit Kate 16.04.2 KDE Frameworks 5.23.0 Qt 5.7 xcb wm
The read-only flag we have and "read-only" on disk have no connection. Therefore to reset it makes no sense, if you mark a document as read-only that sticks until you decide otherwise, per design. For "read-only" on disk, we should show some indication, too, which would go away on save as, but that is bug 366727.