I think it would make sense to add a config for enabling/disabling redirection behavior on the framework.router config. This config would get passed to the PhpMatcherDumper.
For frontend apps it's usually the best to enable that behavior for good user experience. But APIs might want to disable that because often clients cannot deal with redirections anyway.
The other advantage is that this allows to compare the Symfony router with other routers that don't support redirections. Otherwise it skews the benchmark result for non-matching routes.

add a config for enabling/disabling redirection behavior

I'm not sure it makes sense to configure this globally, that's my main objection. Then about adding a per-route option, it doesn't make sense either IMHO: API clients should just hit the correct URL. Trailing slash is focused at browsers and users typing in the address bar. For benchs, I wouldn't care much :)

I would also suggest to enable / disable redirection behaviour by route.
Direct from xxx to xxx/ should be enabled by default (otherwise it is a bc break) but for an api I would have the possibility to disable the redirects.
On the other hand, if I use urls without a trailing slash, I would prefer the possibility to enable the redirect from xxx/ to xxx.

routes.yaml

controllers:
    resource: '../src/Controller/'
    type: annotation
    prefix: /{_locale}

    add_trailing_slash: true|false // true by default
    remove_trailing_slash: true|false // false by default

it would be also great if this behaviour is available in the @route Annotation

    /**
     * @Route("/xxx", name="my_route", remove_trailing_slash=true)
     */
    public function xxxAction()

possibility to enable/disable the redirect from xxx/ to xxx

@sebastianblum I would MUCH prefer NOT adding any new option to teach to anyone. If you write an API, just hit the correct path, as previously said.

@nicolas-grekas I totally agree and I prefer the urls without trailing slash and I don't need the redirect from xxx/ to xxx. If symfony implements a configuration option, I would disable it.

In my opinion, we can ignore the feature of this pr and if users need the redirect from xxx/ to xxx, they can configure it using the redirect controller.

Maybe we should deprecate the auto redirect from xxx to xxx/ and remove this functionality in symfony 5. Like the other case, these redirects are only a few redirects in routes.yml.

Thank you Nicolas.