Blender 2.7x used to have F8 to reload scripts.

As an add-on developer I rely on reloading the scripts a lot and having to press F3 and search for "Reload Scripts” every time I update the add-on I develop and that's pretty much inconvenient IMHO. I tried to add shortkey in the keymap section of the settings but it seems that 'Reload Scripts' doesn’t exist in the list of operators that can be attached to shortkeys.

Q: Is there any way to get F8 back? And in general, how do I go about adding a shortkey to any other operator that is not listed in the keymap section?

New shortcuts can be added in the preferences Edit > Preferences > Keymap.

With the keymap tab open, navigate to the Window category of shortcuts. Click the Add New button to create a new shortcut. The operator for reloading scripts is bpy.ops.script.reload() therefore the identifier for the shortcut is script.reload. Click on the shortcut field and press F8 to set the key binding.

In the default keymap for Blender 2.80 there is no existing key binding on F8 which means you can add the shortcut without any problem.

but you could try with these commands,

from the script-editor or console-window:

import bpy