Using map icons from image/url path breaks view after upgrade

Thanks for reporting your issues @cianevans ... but sorry I cannot reproduce your issues, and this is not a bug at all.
On this side the Leaflet module 2.1.20 release (upgrade to it eventually ...) works as designed.

As confirmation of that just consider that this Leaflet demo page: http://www.geodemocracy.com/geofield_d8/web/geoplaces-map-leaflet
is indeed working with Map Icon as "Icon Image Url/Path" with a token (@see the screenshot),
and double checking also other settings all works fine on my side ...

Your should have some issues with your personal setup, or may be some JS conflict with some custom or other contrib module ... all on your side.

I have encountered the same problem. A map with 800 elements "clogs" the server.
Cause: in LeafletService::setFeatureIconSizesIfEmptyOrInvalid() you call @get_headers($fileUrl) and $this->fileExists() for each Map-Element.
On our Heavy-Traffic site this caused the view to take 15s for 100 Elements and ends with a white screen on 800 elements (on local environment it runs fine).

I use STATIC, to avoid calling often the system-calls which solved the problem at our site:

   public function setFeatureIconSizesIfEmptyOrInvalid(array &$feature) {
    if (isset($feature["icon"]["iconSize"])
      && (empty(intval($feature["icon"]["iconSize"]["x"])) && empty(intval($feature["icon"]["iconSize"]["y"])))
      && (!empty($feature["icon"]["iconUrl"]) && $this->fileExists($feature["icon"]["iconUrl"]))) {
      $iconSize = $this->imageSize($feature["icon"]["iconUrl"]);  // so I can use STATIC here too
      $feature["icon"]["iconSize"]["x"] = $iconSize[0];
      $feature["icon"]["iconSize"]["y"] = $iconSize[1];
    }

    if (isset($feature["icon"]["shadowSize"])
      && (empty(intval($feature["icon"]["shadowSize"]["x"])) && empty(intval($feature["icon"]["shadowSize"]["y"])))
      && (!empty($feature["icon"]["shadowUrl"]) && $this->fileExists($feature["icon"]["shadowUrl"]))) {
      $shadowSize = $this->imageSize($feature["icon"]["iconUrl"]); // not "shadowUrl" !!
      $feature["icon"]["shadowSize"]["x"] = $shadowSize[0];
      $feature["icon"]["shadowSize"]["y"] = $shadowSize[1];
    }
  }


  /**
   * Check if a file exists. Uses STATIC to avoid massive calls of '@get_headers'
   *
   * @param string $fileUrl
   *   The file url.
   *
   * @return bool
   *   The bool result.
   * @see https://stackoverflow.com/questions/10444059/file-exists-returns-false-even-if-file-exist-remote-url
   *
   */
  public function fileExists($fileUrl) {
    static $fileUrls = [];
    if (!isset($fileUrls[$fileUrl])) {
      $file_headers = @get_headers($fileUrl);
      if ((stripos($file_headers[0], "404 Not Found") == 0)
        && (stripos($file_headers[0], "403 Forbidden") == 0)
        && (stripos($file_headers[0], "302 Found") == 0 && isset($file_headers[7]) && stripos($file_headers[7], "404 Not Found") == 0)) {
        $return = TRUE;
      }
      else
        $return = FALSE;
      $fileUrls[$fileUrl] = $return;
      return $return;
    }
    else
      return $fileUrls[$fileUrl];
  }

  /**
   * Uses STATIC to avoid massive calls of 'getimagesize'
   * @param $fileUrl
   *
   * @return array|false|mixed
   */
  function imageSize($fileUrl) {
    static $fileUrls = [];
    if (!isset($fileUrls[$fileUrl])) {
      $iconSize = getimagesize($fileUrl);
      $fileUrls[$fileUrl] = $iconSize;
      return $iconSize;
    }
    return $fileUrls[$fileUrl];
  }

Thanks anyway for the great module !!