Even if this could be done in some way so that it would work with ACF image fields, there still would not be any way to prevent images being “removed in an over-enthusiastic cleanup.” WP uses the
post_parent in the
_posts table to mark an image uploaded to a post. WP has no built in mechanism for uploading or adding attachments to any other type of object. This is something that’s not likely to change, I doubt that you’ll ever see a built in WYSIWYG on a term or anywhere else in WP for that matter. Also, the
post_parent field requires an integer that is assumed to be another post ID, so anything created would be a work-a-round that would be outside of anything that WP would recognize.
There is one solution that I just thought of. There is the ability for ACF to save field values of options pages as related to a “post” (see the ‘post_id’ argument https://www.advancedcustomfields.com/resources/acf_add_options_page/) and I think that this will make it work the same, I just tested this and I know it works. The thing here is that you need posts, of some type, to store the values on and this means creating some type of post type for holding the options fields. My plugin that lets you create options pages using an admin interface will also let you save fields to the post of the options page settings… https://wordpress.org/plugins/options-page-admin-for-acf/
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