justintadlock/plugin.php

## plugin.php
<?php

// This filter specifically checks whether a theme has a `changelog` template.
// If not, it looks for a fallback template in the plugin.
add_filter( 'template_include', function( $template ) {

	// I'm just testing a query string here for demonstration/testing
	// purposes, but you'll want to add a check for your registered query
	// var with WordPress. For now, you can test with `?changelog=anything`.
	if ( ! isset( $_GET['changelog'] ) ) {
		return $template;
	}

	// Custom hierarchy of templates.
	$templates = [
		'changelog.php',
		'index.php'
	];

	// First, search for PHP templates, which block themes can also use.
	$template = locate_template( $templates );

	// Pass the result into the block template locator and let it figure
	// out whether block templates are supported and this template exists.
	$template = locate_block_template( $template, 'changelog', $templates );

	// If the template is found, use it. Otherwise, fall back to the plugin
	// template. It's worth noting that the wrapping block template markup
	// and block processing wouldn't be available to the plugin template.
	// The plugin would have to handle all of this on its end and probably
	// add a few actions/filters that WP would normally add.
	//
	// I'd recommend digging through `wp-includes/template-canvas.php` and
	// the `locate_block_template()` function for building this fallback.
	return $template ?: '/path/to/plugin/fallback-template.php';
} );

// Filter the default template types, adding your own.
add_filter( 'default_template_types', function( $templates ) {
	return array_merge( $templates, [
		'changelog' => [
			'title'       => 'Changelog',
			'description' => 'A custom changelog template.'
		]
	] );
} );
	<?php

	// This filter specifically checks whether a theme has a `changelog` template.
	// If not, it looks for a fallback template in the plugin.
	add_filter( 'template_include', function( $template ) {

	// I'm just testing a query string here for demonstration/testing
	// purposes, but you'll want to add a check for your registered query
	// var with WordPress. For now, you can test with `?changelog=anything`.
	if ( ! isset( $_GET['changelog'] ) ) {
	return $template;
	}

	// Custom hierarchy of templates.
	$templates = [
	'changelog.php',
	'index.php'
	];

	// First, search for PHP templates, which block themes can also use.
	$template = locate_template( $templates );

	// Pass the result into the block template locator and let it figure
	// out whether block templates are supported and this template exists.
	$template = locate_block_template( $template, 'changelog', $templates );

	// If the template is found, use it. Otherwise, fall back to the plugin
	// template. It's worth noting that the wrapping block template markup
	// and block processing wouldn't be available to the plugin template.
	// The plugin would have to handle all of this on its end and probably
	// add a few actions/filters that WP would normally add.
	//
	// I'd recommend digging through `wp-includes/template-canvas.php` and
	// the `locate_block_template()` function for building this fallback.
	return $template ?: '/path/to/plugin/fallback-template.php';
	} );

	// Filter the default template types, adding your own.
	add_filter( 'default_template_types', function( $templates ) {
	return array_merge( $templates, [
	'changelog' => [
	'title' => 'Changelog',
	'description' => 'A custom changelog template.'
	]
	] );
	} );