1. What kops version are you running? The command kops version, will display
this information.

1.18.1

2. What Kubernetes version are you running? kubectl version will print the
version if a cluster is running or provide the Kubernetes version specified as
a kops flag.

1.18.9

3. What cloud provider are you using?

aws

4. What commands did you run? What is the simplest way to reproduce this issue?

kops upgrade cluster on a kops >=1.18.0 cluster

5. What happened after the commands executed?

I1007 16:04:36.843987  775287 upgrade_cluster.go:198] Custom image (099720109477/ubuntu/images/hvm-ssd/ubuntu-focal-20.04-amd64-server-20200907) has been provided for Instance Group "nodes"; not updating image

6. What did you expect to happen?

kops should handle upgrades from images listed in the channels

**7. Please provide your cluster manifest. Execute

8. Please run the commands with most verbose logging by adding the -v 10 flag.
Paste the logs into this report, or in a gist and provide the gist link here.

9. Anything else do we need to know?

Message is triggered when the image name doesn't contain "kope.io". It should probably check for the images listed in the channels/stable file instead

See:

stable channel now uses the 099720109477/ubuntu/images/hvm-ssd/ubuntu-focal-20.04-amd64-server-20200907 image, which triggers the warning
https://github.com/kubernetes/kops/blob/56f3cb33481ab7c1d073ddaac6e4873b0b264249/channels/stable