Updated: Comment #0
Problem/Motivation
There are currently two .tpl.php files in core and they are only used for automated tests that ensure PHPTemplate still works. Everything else has been converted to .html.twig.
The Twig conversion patches committed as part of #1987510: [meta] Convert all core *.tpl.php templates to Twig as singular patch got rid of a lot of references to .tpl.php but not everything.
Proposed resolution
Create a patch that updates all relevant occurrences (in documentation) of .tpl.php to .html.twig. There is a separate issue for the non-documentation Views changes: #2049209: .tpl.php should be .html.twig or theme engine agnostic in Drupal\views\Plugin\views\display\DisplayPluginBase
Example of what we need to update, from default.settings.php:
/**
* A custom theme can be set for the offline page. This applies when the site
* is explicitly set to maintenance mode through the administration page or when
* the database is inactive due to an error. It can be set through the
* 'maintenance_theme' key. The template file should also be copied into the
* theme. It is located inside 'core/modules/system/maintenance-page.tpl.php'.
* Note: This setting does not apply to installation and update pages.
*/
# $conf['maintenance_theme'] = 'bartik';Remaining tasks
Patch needs to be created
User interface changes
n/a
API changes
n/a
Related Issues
#1987510: [meta] Convert all core *.tpl.php templates to Twig as singular patch
#1757550: [meta] Convert core theme functions to Twig templates