My question is this availability is Microsoft responsibility, right ?
Yes, Resource Availability is Microsoft's responsibility.
how to find out root cause of why this availability is impacted . why it is getting below 80 percent.
Open Resource in the portal and go to Resource Health tab. You should see the details if there was a problem with the resource.
If the issue was face worldwide or for any specific region, they are also reported here with root cause: https://status.azure.com/en-us/status/history/
Is there anything to do from my side ?
Check resource health or Azure status website. Optionally you can also raise support request from the portal.
One more question : what does it mean by "Azure Data Factory V2 - Mitigated ".
If you saw this on Azure status website, it means that there was some issue/bug with ADFv2 and now it has been fixed/mitigated
Please don't forget to Accept Answer and Up-vote if the response helped -- Vaibhav