Azure Backup supports backup of 64-bit Windows server operating system from Windows Server 2008.
Azure Backup supports backup of 64-bit Windows 10 operating system.
Azure Backup supports backup of 64-bit Ubuntu Server operating system from Ubuntu 12.04.
Azure Backup supports backup of VM that are shutdown or offline.
Reference:
https://docs.microsoft.com/en-us/azure/backup/backup-support-matrix-iaas
https://docs.microsoft.com/en-us/azure/virtual-machines/linux/endorsed-distros