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The majority of code snippets I use this code block macro for are JSON and YAML snippets. I think it's kind of lame that in 2020 only XML is properly supported by this block and not JSON and YAML.
When is Atlassian going to brush this one up?
You can add code block highlighter brushes in Confluence Administration -> Configure Code Macro. Below are links to brushes that you can use to add highlighting for JSON and YAML.