This is a composer package to integrate the redirects from version 9 into version 8.7.
Since this feature needs some other patches for the core, the composer patches packages is used to patch the core on composer install.
Read (https://github.com/cweagans/composer-patches#user-content-allowing-patches-to-be-applied-from-dependencies) to allow patches to be applied from dependencies, if you don't already use composer patches.
After running composer require mabahe/typo3-core-redirects the patching will fail, because the package will
be installed after typo3/cms.
To fix this run composer install.
Additionally this package contains a patch to be able to set all domains of a site root dynamically using a placeholder as domain. This will hopefully be available in version 9 soon, too.
From realurl redirects https://gist.github.com/Mabahe/99a92f9b25878cfec3af9f400bde6568