The package provides some Symfony resources for bundles that want to be compatible with both Symfony 4 and 5:
The InputBag class was added to the HttpFoundation component in Symfony 5.
The polyfill will save you a lot of if (class_exists(InputBag:class)) checks
by providing the InputBag class in Symfony 4 as well.
The TranslatorInterface has been moved from the Translation component to the
Contracts component in Symfony 5. The polyfill restores the old interface in
Symfony 5.
If you want your Contao extension to be compatible with multiple Contao
versions (e.g. 4.9 to 4.13), you will also have to be compatible with both
Symfony 4 and 5. Require the polyfill in your composer.json in this case.
Contao is licensed under the terms of the LGPLv3.
Visit the support page to learn about the available support options.