Sub-directories in module's templates directory
You just need to specify the path to the template in hook_theme(), eg

function MYMODULE_theme() {
  return [
    'list_block' =&gt; [
      'variables' =&gt; [],
      // Underscores are not converted to dashes automatically in this case.
      'template' =&gt; 'account/list-block',
    ],
  ];
}