Share taxonomy between user and posts?

No you can’t, for a simple reason:

This term says that ID 1 is in the food category, but is that user 1 or post 1?

The ID provides no context, it’s an object ID. As a result, there’s no way to tell if that ID is a post ID or a user ID. This is why multiple post types can share a taxonomy as they’re all posts, but you can’t mix posts and users

Sidestepping the Issue

Use 2 taxonomies and a little magic! Create one for users, and one for posts, where every term exists twice in both with the same name/slug. Then use hooks and filters to create and update terms as they’re modified in either taxonomy.

I anticipate that this might lead to the question of how to query for both at the same time. I’m afraid that’s not possible. Instead, get_objects_in_term can get you the relevant users, and WP_Query can fetch the posts

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