I wonder why I didn't make customizer a Class in StructuredLoggingJsonProperties

I wonder why I didn't make customizer a Class in StructuredLoggingJsonProperties

Do you mean in this context? If so, I was also confused about why java.lang.Class wasn't simply used here.

    {
      "name": "logging.structured.json.customizer",
      "type": "java.lang.String",
      "description": "The fully qualified class name of a StructureLoggingJsonMembersCustomizer"
    },

Exactly, I've made that change in #43202 which should remove the need for additional metadata. Thanks for finding it!

additional-spring-configuration-metadata.json in spring-boot/spring-boot-project/spring-boot/src/main/resources/META-INF still contains the following:

    {
      "name": "logging.structured.json.customizer",
      "type": "java.lang.String",
      "description": "The fully qualified class name of a StructuredLoggingJsonMembersCustomizer"
    }

Should it be updated?

    {
      "name": "logging.structured.json.customizer",
      "type": "java.lang.Class",
      "description": "The fully qualified class name of a StructuredLoggingJsonMembersCustomizer"
    }

Thanks @nosan, I've opened #43916