For all instances game is saved to "user://saves" which means different things in different OS. This system is a part of godot engine and should probably be addressed there. I'm neither good nor familiar with mac file structure so unless you have a ready solution which wouldn't impact all other instances, I'm afraid there's not much I can help with.
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that's unfortunate then.
The way the sandboxing mechanism on mac works is that it denies read and write access to all files except those in the directories specified by the profile provided by itch, meaning linking via hardlinks or symbolic links would not work since they too would be files.
If the godot engine has no option to specify different directories for save files on different systems then yes, should probably be brought to their attention. Apart from that saving stuff under $HOME isn't even common for mac applications in the first place. Most save their stuff under $HOME/Documents or in that Application Support folder under $HOME/Library.
Anyway, respect to you for taking all that time to talk so much to me here, I would have never expected to get such a direct line to the developer!
Technically you could specify nearly any location and check the user's OS before that. The main problem, as I said, I don't have mac so I can't really figure out how user system differs from normal, and even if I could, there's no good way to test it out personally. So if someone capable gets it done, I'll include the fix for it.
No problem, feedback is always a great way to work on your faults.