I recently update to Symfony 4.4 and had to work through a few deprecations. Some were straight forward, some were not. This Twig one was not:
The "twig.exception_controller" configuration key has been deprecated in Symfony 4.4, set it to "null" and use "framework.error_controller" configuration key instead.
This was resolved by adding the following to config/packages/twig.yaml:
exception_controller: null