In 3.4, the only Windows-specific settings I see in config-main.def is
[Keys]
default= 1
name= IDLE Classic Windows

I have presumed that this is somehow changed on other systems. If not, that is an issue.

Config-keys.def actions (should) have a Meta variant when Alt is used.
close-window=<Alt-Key-F4> <Meta-Key-F4>

If 'Alt' were automatically changed to 'Meta' on Mac, before sending to tk, the Meta entries could be removed, with a note in the key-setting dialog about the auto translation. The file would be easier to read; setting keys would then be easier; and Macs would automatically get key bindings.

(I notice that some actions have two Alt bindings and only one Meta binding. Is that because Meta in not value for all uses of Alt?)