Task priority is supported in company and deal based workflows. Contact workflows are being worked on now.
Could you explain why you want a label for MEDIUM and LOW priority tasks? How do you know when a task is HIGH priority vs. MEDIUM or LOW? And what's the difference between a task that is MEDIUM or LOW priority and NOT a priority?
Tasks does support sorting by priority and filtering by priority today.
Scott from the product team here. We recently released the option to set a priority level on a task to all users. When creating tasks, you'll now have the option to label the task as "high" priority. When working in tasks, you can now also sort and filter by task priority.