How to set 'image tag mutability' using CloudFormation?
The ImageTagMutability property in ECR is not currently supported in CloudFormation, as verified by experimentation (nor is it part of the LifecyclePolicy property). There is sometimes lag between feature availability in CloudFormation, and the mutability flag was only introduced in late July.

This comes up elsewhere in the corners of AWS, including:


AWS Global Accelerator - this entire service is not provisionable using CloudFormation.
AWS WAF geo match rules cannot be created in CFN.