Is single plugin files in the plugin root folder deprecated?
https://github.com/getkirby/kirby/blob/master/kirby.php#L432
No, why should it be?
It’s not in the docs anymore?
https://getkirby.com/docs/developer-guide/plugins/hello-world
That document hasn’t changed in a year, you can still just add a simple file, I’ve always been doing it for a collection of methods. You can easily test that yourself.