#Needs backport to D7

There is callback_allowed_values_function() in api docs, and various methods are marked as this.

If any are missing this, it's a specific docs issue rather than this global one, which I think we can say is done.

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The Needs Review Queue Bot → tested this issue. It no longer applies to Drupal core. Therefore, this issue status is now "Needs work".

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Based on the issue summary, comments and related issues the immediate problem is limited to single select elements and client side validation. If I missed something, let me know and I can update the issue accordingly

Refactoring the _none value itself is being worked on in 📌 Convert '_none' option to a constant and deprecate form_select_options() Needs work

idebr → changed the visibility of the branch 1585930-options-module-uses to hidden.

To minimize the BC break, I suggest we map the _none option to the Select #empty_option and set #empty_value to an empty string in a process callback.

This implementation allows existing options to remain unchanged; only client side logic is impacted (eg. JavaScript)

And this was fixed.

This was committed to 8.0 and not fixed in 7 so I am updating the version. Also, updated credit, which is always challenging on an older issue with lots of comments.