For what it’s worth, 1.21 includes an update to Twig (from v1.41 to v1.44), which has broken custom code on a number of sites for me. Themes that make customizations to Twig are at risk for this. (For example, I had a custom implementation of \Twig\Loader\LoaderInterface that raises a fatal error in Timber 1.21 because the interface definition in Twig changed.)
