Description
There are various reasons why an app may need to logout a user without the user having to be redirected to the logout page, and currently there is no easy way to do this without copying some code from core which is subject to change and thus may leave one with outdated logic.

Example
Related to #40662, IMO a great API to have would be Security::programmaticLogout(Request $request)

This would figure out which firewall is relevant for the given $request, and trigger a logout event on that firewall's event dispatcher.

Why?
By digging through internals, it appears the best way to do this is the following:

        $logoutEvent = new LogoutEvent($request, $this->tokenStorage->getToken());
        $this->eventDispatcher->dispatch($logoutEvent);

        $response = $logoutEvent->getResponse();
        if (!$response instanceof Response) {
            throw new \RuntimeException('No logout listener set the Response, make sure at least the DefaultLogoutListener is registered.');
        }

        $this->tokenStorage->setToken(null);

        return $response;

This is copied from

On top of that, some added complexity one has to figure out is that the event dispatcher here isn't just any event dispatcher, but you have to inject the correct event dispatcher for the firewall you want to do the logout on, and so you have to define that in the service yourself.. Firewall-specific event dispatchers are mentioned in passing in the docs but it's not exactly a common concept so this is all overall not great DX, and having an easy and more visible place to do this would be nice I think.