While this is a good thought it wouldn’t be wise to implement this for one reason; Not everyone uses WPMU as master site > sub sites. Many uses it as just a network where the main site doesn’t really have that kind of connection/sync to the others.
HOWEVER, I think a tweak of this could be a great idea. The ACF JSON files could be inherited by a childtheme from the parent theme. That way you’d be able to use it if you like and it follows the WordPress way of inheritance by childthemes. It would however require some way to be able to override that a field group shouldn’t be inherited or deleted for the childthemes.
Does that sound good to you?
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