As explained in Configuring issue-level security, the issue security can be done based on A (multi-)user or (multi-)group picker custom field.
Using it in case when the Custom field has value in the Story, but not in Subtask:
- When viewing the issue - Subtasks are hidden for users who have no permission.
- On the board - Subtasks are showing for users who have no permission.
- Create Custom field of type Group picker
- Create Issue Security Scheme and add "Group custom field value" to previously created custom field
- Associate this Issue type scheme with the project
- Note that the subtask are hidden for users that don't have permission to see the subtasks in View Issue
Subtasks are not shown on board
Subtasks are showing on board
Tried adding a different Visible to Group for the subtasks as well but, it it still shown on the board.