We likely missed this because it's a lazy builder callback that, when big pipe is enabled, is too late to be reported as a deprecation. This will also require to update the .info.yml to require ^8.8 because it was introduced in https://www.drupal.org/node/2966725

Problem/Motivation The current theme system is a complicated mix of sub systems that pass data back and forth to one another in ways that confuses new users, and causes bugs. @see [#939462] We've solved many of these problems in Drupal 8 by re-architecting the theme layer, but sadly were not able to get everything done before code freeze. We've stopped using the preprocess hooks, and have deprecated them, but were not able to remove them in the Drupal 8 release cycle. @see [#2004872]

This is blocked by [#2869859]. Problem/Motivation All changes to $variables that will be printed in templates currently happen in the same place, preprocess. Things that happen in preprocess include both creating initial variables and then altering those variables later. Proposed resolution