It's sometimes possible to restore a deleted service account using the undelete command.
To undelete a service account, the caller must be granted the iam.serviceAccounts.undelete permission on the deleted service account. This permission is included in the Owner (roles/owner) primitive role.
The deleted service account's unique ID must be provided when using the undelete command.
Use the serviceAccounts.undelete()
POST https://iam.googleapis.com/v1/projects/[PROJECT-ID]/serviceAccounts/[ACCOUNT-UNIQUE-ID]:undelete
You would get 200 response code with details about the restored service account, on successful recovery of the account.
POST https://iam.googleapis.com/v1/projects/[PROJECT-ID]/serviceAccounts/[ACCOUNT-UNIQUE-ID]:undelete
Hope this helped!!
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