Interestingly they can’t be switched off or uninstalled from wp-admin, and are automatically loaded, similar to putting the code in your functions.php file. This makes them ideal for use if your theme requires some functionality, such as ACF.
There are a few caveats, such as you can’t update the plugin and WP won’t descend into subdirectories looking for them. But these can be overcome.
I wonder if anyone has tried to port ACF to use as an muplugin, and if it worked / was it a good idea?
I have recently moved ACF into mu-plugins on a rather high-traffic site. I’ll let you know about the problems as they arise. I simply put the advanced-custom-fields-pro directory into the wp-content/mu-plugins directory and added this modest file to that directory as well: