Case Activity does't respect 'Do not notify assignees for'?
It's not intentional but it's come up before. See https://lab.civicrm.org/dev/core/issues/1582 and https://lab.civicrm.org/dev/core/issues/1649

UPDATE:
@pradeep-nayak submitted a PR https://github.com/civicrm/civicrm-core/pull/16800 so it will restrict for case activities too in version 5.25.