Root .theme file
Call to deprecated function file_unmanaged_save_data(). Deprecated in drupal:8.7.0 and is removed from drupal:9.0.0. Use Drupal\Core\File\FileSystemInterface::saveData().
file_unmanaged_save_data()
Drupal\Core\File\FileSystemInterface::saveData().
Info file:
Add core_version_requirement: ^8 || ^9
core_version_requirement: ^8 || ^9
(Checked the Umamy theme.)