Proj4js 2.x support with OpenLayers 2.x. Open the brower console to see the result

<!doctype html>
<html lang="en">
  <head>
    <script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/proj4js/2.3.3/proj4.js"></script>
    <script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/openlayers/2.13.1/openlayers.js"></script>
    <script type="text/javascript">
      //Snippet comes from https://osgeo-org.1560.x6.nabble.com/OL-2-13-1-latest-Proj4js-td5081636.html
      window.Proj4js = {
        Proj: function(code) {
          return proj4(Proj4js.defs[code]);
        },
        defs: proj4.defs,
        transform: proj4
      };
      // Definition comes from https://epsg.io/31466/
      proj4.defs["EPSG:31466"] = "+proj=tmerc +lat_0=0 +lon_0=6 +k=1 +x_0=2500000 +y_0=0 +ellps=bessel +towgs84=598.1,73.7,418.2,0.202,0.045,-2.455,6.7 +units=m +no_defs";
      console.log(Proj4js.defs["EPSG:31466"]);

var proj_wgs84 = new OpenLayers.Projection("EPSG:4326");
      var proj_31466 = new OpenLayers.Projection("EPSG:31466");
      // Center for projection EPSG:31466
      var point = new OpenLayers.LonLat(2769212.70, 5678724.61);
      var new_point= point.transform(proj_31466, proj_wgs84);
      console.log(new_point.lat, new_point.lon);
    </script>
  </head>
  <body>
  </body>
</html>

Updated missing url //cdnjs.cloudflare.com/ajax/libs/openlayers/2.13.1/OpenLayers.js to //cdnjs.cloudflare.com/ajax/libs/openlayers/2.13.1/openlayers.js

https://cdnjs.cloudflare.com/ajax/libs/proj4js/2.3.3/proj4.js

https://cdnjs.cloudflare.com/ajax/libs/openlayers/2.13.1/OpenLayers.js