On October 9, Juliette Reinders Folmer announced on the core WordPress blog that WordPress 5.3 will use the spread operator. The spread operator was one of the new features made available in PHP 5.6, a version released in 2014.
WordPress abandoned PHP 5.2 – 5.5 with the release of WordPress 5.2. This means the core team can start taking advantage of relatively new features, or at least 5-year-old features. For plugin and theme developers who maintain the same minimum version support as WordPress, they can also start exploring this feature.