metadata customTemplates

A way to add data to the custom templates placed in the /templates folder:

{
	"$schema": "https://schemas.wp.org/trunk/theme.json",
	"version": 2,
	"customTemplates": [
		{
			"name": filename",
			"title": "Template Name",
			"postTypes": [
				"page"
			]
		}
	],
	"settings": ...
}

Source: https://developer.wordpress.org/block-editor/how-to-guides/themes/global-settings-and-styles/#customtemplates