I'm proposing to deprecate the RouteCollectionBuilder added in #15778 for several reasons:

1. it does not support any of the newer features of all the other loaders like i18n routes ([Routing] Implement i18n routing #26143) or imports without trailing slash ([Routing] allow no-slash root on imported routes #26284)
2. it's not documented anywhere in the symfony docs
3. it seems obsolete since introducing RoutingConfigurator ([Routing] Add PHP fluent DSL for configuring routes #24180) which covers the same use-cases
4. It's only used by the MicroKernelTrait forcing people two learn another API but at the end developers don't care much because they likely only import routes from files with a glob pattern as provided by flex/reciepes

To solve 4) we can deprecate the Symfony\Bundle\FrameworkBundle\Kernel\MicroKernelTrait::configureRoutes and provide a different method that accepts a RoutingConfigurator instead.