cf. T133541: [RFC] Limit option (or provide alternative) to change the priority of tickets to project members or T819: Restricting modification of tasks when they enter sprints (both not exactly the same but closely related).

Folks changing a task's Priority value (often without bad intentions but "the field was visible so I thought I can") regularly comes up as an issue faced by teams whose workflows rely on the Priority field.

Upstream Phorge (Phabricator) code itself has no further differentiation in the Maniphest (=tasks) application settings: The default Maniphest Edit Policy covers all fields of a task - Priority field and any other fields.
To some extent this was mitigated by the introduction of forms allowing to disable or hide specific UI fields, which however still does not solve the situation as a (Create or Edit) form configuration cannot be bound to a specific user group - it's all users or no users affected by the form configuration, once a task has been created via a form.

I was quite reluctant in the past to restrict editing the Priority field. After having seen vandalism (though that's only weakly related here) and repeated tension over the years I've started to consider this an option.
It would slightly reduce UI complexity for Phab newcomers not aware of its social conventions.
It would reduce workflow collisions, see e.g. https://phabricator.wikimedia.org/T14396#9680109 or https://phabricator.wikimedia.org/T362986#9730145 or https://phabricator.wikimedia.org/T365323 or https://phabricator.wikimedia.org/T365315#9811246 (random latest examples).