Problem/Motivation
#2511548: Add a "context" array variable to all theme hooks and "#context" array property to all elements to provide optional contextual data introduced the "context" concept into the theme system, where this kind of metadata should actually live, not poluting the $variables array.
For BC reasons, twig_render_template() is allowed to be invoked without passing the third $context parameter and its value is derrived from $variables where the same properties exist in 8.x. Because these properties are also deprecated, reliance on using them will be removed in 9.x as well.
Proposed resolution
Remove BC support.
Remaining tasks
TBD
User interface changes
TBD
API changes
TBD
Data model changes
TBD
Release notes snippet
TBD