I had to shut down a node in December, I did a graceful exit, and et went fully completed.
Can I follow the heldback amount somewhere, or when can I expect the payout ?
I am about to start up a new node, but want to feel assured that the life-cycle for a node is fully working first. 
Hello @TommyBdk ,
Welcome to the forum!
If you did not opt-in for zkSync or Polygon, your payout is a subject of the minimum payout threshold. And if your undistributed amount is not greater than $66 it will waiting for a lower fee in the next payout period (first two weeks of every month) or opting in zkSync or Polygon, which does not have a minimum payout threshold and also zkSync have a 10% bonus.
To opt-in you need to have an identity of the exited node - you can run it once after changing the configuration for zkSync or Polygon, to send a message to the satellites that you changed your wallet options for this node and want to receive payout via L2.
If you do so, your undistributed amount will be sent via choosen network to your wallet address in the next payout period.
…and where can I see if my amount is under $66 ?
On the node’s dashboard.
Or, if you still have databases, with an Earnings calculator (Update 2022-01-13: v11.0.0 - Detailed earnings info and health status of your node, including vetting progress)
If you deleted your identity, then you cannot check its earnings. We can’t say you even via support request, because it will be difficult to prove that you are the owner of the node. However, you can do that if you can sign a message with a private key of your wallet.
Okay, I am gonna see if I can spin up a docker with that identity.