As you rightly notice you need to use the formatter function for data labels. Use below code to check if the point is the last one.

series: {
  dataLabels: {
    enabled: true,
    formatter: function() {
      var seriesPoints = this.series.points;
      if (this.point === seriesPoints[seriesPoints.length - 1]) {
        return 'Last label point';

Live demo:

API Reference:

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