Hi Dhairya
The legacy notifications refer to notifications that are not part of centralised notifications..
Legacy notifications are being converted to centralised notifications gradually and some of these have not been converted yet such as some of the activity notifications.
Each legacy notification has been created independently and may be triggered by different settings , Centralised notifications aims at making this consistent and centrally managed.
See https://totara.help/docs/what-are-centralised-notifications for more details of centralised notification development
regards