Autobahn and Composer compatible version of the Bedrock Autoloader for mu-plugins.

Install it with

composer require mrgrain/bedrock-autoloader

and set WPMU_LOADER in the autobahn.json file of your project:

{
    "config": {
        "WPMU_LOADER": "bedrock-autoloader/bedrock-autoloader.php"
    }
}

All kudos belong to the lovely people behind Bedrock.
And a little cheer goes to @mokomokoy for adjusting the autoloader for Autobahn.