Retrieve Role Assignments for a list:
/_api/web/lists/getbytitle('list title')/roleassignments?$expand=Member/users,RoleDefinitionBindings
Retrieve Role Assignments for a listitem:
/_api/web/lists/getbytitle('list title')/items(id)/roleassignments?$expand=Member/users,RoleDefinitionBindings
Check if site/list/listitem has a unique permission:
/_api/web/HasUniqueRoleAssignments
/_api/web/lists/getbytitle('list title')/HasUniqueRoleAssignments
/_api/web/lists/getbytitle('list title')/items(id)/HasUniqueRoleAssignments
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