Usually deleting user will be tracked in Azure AD Audit logs. By default, if you have a subscription you can see about last 30 days of logs.
If you do not have any subscription linked to directory, then only last 7 days logs are stored.
If you are looking for a deleted user audit log for year 2018 or 2019 then it is not possible.
If you had created a backup for logs from Azure AD then you can search for user deletion in those logs.
About notifications when user is deleted, to get notification you will have to configure notifications. You can refer below article for the same,
https://learn.microsoft.com/en-us/graph/webhooks
Let me know if you have any further questions.
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