Always ON Uninstallation I have a request to uninstall one of the test instance which is part of Always ON. I googled around for a clean decommission but did not find steps for Always ON Instance uninstallation. Would be great if Steps can be shared here or any link if you are aware of. As per me I would :


Take database backup
Take note of Availability group listener and database involved.(For future if they want it back)
Remove the Databases from the AG.
Remove the AGs
Uninstall the instance as a normal standalone instance.(For all nodes).


Please add or suggest if this needs any changes.
Do you have any replications setup? 
Maybe you also want to include a step to remove those incoming or outgoing replications  after your step 2?

It should be fine even if you omit this step, but it might generate unnecessary alerts (if you have set monitoring for replication) to the oncall.