4.4 to 5.0
An explanation of the code changes for each version of Joomla. If you follow from the version of your current code until the version you want to support you should come across all the changes you need to make.
Compatibility Plugin
Most of the code deprecated in Joomla! 3.x has been removed. Some deprecations have been moved to the Compatibility Plugin.
Replacement of Factory::getUser
Factory::getUser() is deprecated since Joomla 4.0. To replace it, models, views, form fields and tables can implement the CurrentUserInterface and then is the currently logged in user available through $this->getCurrentUser().
In 5.0 are all the template files (default.php) changed from Factory::getUser() to $this->getCurrentUser().
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