Pretty sure that specific instance is a relic of the past. It was used to import the base class that your plugin class extended. (eg the
class PlgUserJoomla extends JPlugin
joomla.plugin.plugin was a representation for the file in the libraries folder
JPATH_ROOT . "/libraries/joomla/plugin/plugin.php" if I remember right.
jimport('xxx') is currently an alias for
JLoader::import('xxx') and still used for some imports in the Joomla source.
function jimport($path, $base = null)
return JLoader::import($path, $base);
However, the necessity of that I am unsure of. Much of the source auto-loads now either via a namespace or or an alias.
JPlugin is an alias of
JLoader::registerAlias('JPlugin', '\\Joomla\\CMS\\Plugin\\CMSPlugin', '5.0');
Note: As of right now, current version of Joomla is 3.9.13.