Localization works but still get “This plugin is not properly prepared for localization” msg in directory

I figured it out almost immediately after asking the question, after months of frustration. The issue was I was using the load_textdomain function (which was the way I learned it) instead of the apparently newer/favored load_plugin_textdomain.

The working code looks like this

function rsvpmaker_load_plugin_textdomain() {
  load_plugin_textdomain( 'rsvpmaker', FALSE, basename( dirname( __FILE__ ) ) . '/translations/' );
}
add_action( 'plugins_loaded', 'rsvpmaker_load_plugin_textdomain' );