WebKit plugins?

[Po Lu]

Inspecting the source of `xwidget.el', I came across the following
curiosity:

  (when (memq window-system '(mac ns))
    (defcustom xwidget-webkit-enable-plugins nil
      "Enable plugins for xwidget webkit.
  If non-nil, plugins are enabled.  Otherwise, disabled."
      :type 'boolean
      :version "28.1"))

The only place where these are used is in nsxwidget.m, which is only
useful on macOS:

  Lisp_Object enablePlugins =
    Fintern (build_string ("xwidget-webkit-enable-plugins"), Qnil);
  if (!EQ (Fsymbol_value (enablePlugins), Qnil))
    configuration.preferences.plugInsEnabled = YES;

I couldn't find any documentation with respect to what these plugins
are, and what they do.  Could someone explain what these are, and if
and/or why they cannot be used with WebKitGTK+?  Thanks.