Problem/Motivation
twig_init, nyan_cat_init, phptemplate_init are all pointless
Proposed resolution
Call Extension::load() from \Drupal\Core\Theme\ThemeInitialization::loadActiveTheme()
Remaining tasks
Add change record
Improve deprecation notice
User interface changes
None
API changes
Calling THEME_ENGINE_init() is deprecated and will be removed in Drupal 10.
Data model changes
None
Release notes snippet
N/a