Are you sure you have checked all the behaviors of your CloudFront distributions, to make sure the function is not associated anywhere?
If you are confident that is the case, and you've followed all the instructions in the documentation: https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/lambda-edge-delete-replicas.html then I'd recommend opening a case with AWS support to investigate further.
Because the lambda@edge are replicated to edge location, you need to wait for a while. Detache it from CloudFront distribution and wait for a while for AWS to remove replicates.
Normally it should take no more that few minutes. But I also had a situation when I was not able to remove it even after days, then I recommend to contact support and they will remove it for your
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