This error is likely be repro'd out of tests only, but reporting as I see it.
Using toUrl within a Kernel test results in the following uncaught exception.
Drupal\Component\Plugin\Exception\ContextException: The 'entity:user' context is required and not present.
This is causing the tests for Linky project to fail. This may be caused by new code introduced this year in #2942907: Entity system does not provide an API for retrieving an entity variant that is safe for editing, specifically the changes introduced to \Drupal\Core\ParamConverter\EntityConverter::convert