The meaning of the
ONCHAIN status means that the channels it is closed, you are missing 99 more blocks before that your channels if forgetting by c-lightning.
99 Blocks + the first that your transaction is considered on-chain means = 100 blocks that it is the method to say that somethings it is forever in Bitcoin time 🙂
A test that shows the coverage of this test case can be found here
P.S: When your channel is in
ONCHAIN status you can move all the funds around without waiting that this channel will disappear. You can see the transaction or transaction unreserved in your
listfunds RPC command.