I assume it is for the Field button in the editor.

@Quy thanks for the answer but I don't think that's it. Deleting the file has no impact on that button or the resulting modal

You're right. I think it is safe to delete. In it there are untranslated language strings.

That kind of modal is used when there are associations that can be set when editing an item.
For example modal_article for com_content:
https://github.com/joomla/joomla-cms/blob/4.0-dev/administrator/components/com_content/Model/ArticleModel.php#L1107-L1137

1. There is no code to associate fields in the preprocess form of FieldModel
2. I don't even know if it makes sense to associate fields.

ATM, the field type is not used.

As for the Field displaying in the TinyMCE CMS Content dropdown, it depends on the Button - Field editor-xtd and I wonder what it is used for as we can't select a field in the modal produced but only displays the existing fields. Normally though, looking at the $link, it should use that modal.
$link = 'index.php?option=com_fields&view=fields&layout=modal&tmpl=component&context=' . $context . '&editor=' . $name . '&' . Session::getFormToken() . '=1';

It works in J3, but not in J4.

From the Content - Fields plugin:

This plugin allows you to display a custom field which has been inserted with the 'Button - Fields' plugin or using Syntax: {field #} directly into the editor area.
Possible Syntax:

    {field 1} will display the field with the ID 1
    {field 1,foo} will display the selected field using the alternative layout 'foo'.
    {fieldgroup 2} will display all fields within the fieldgroup with the ID 2.

Set to "closed" on behalf of @Quy by The JTracker Application at issues.joomla.org/joomla-cms/25125

Please test PR #25139.

Please test PR #25139.