How can I disable auto login after registration in laravel 8?

First you must create a controller preferably in app\Http\Controllers\Auth called RegisteredUserController, in this controller you must overwrite the method store of the class RegisteredUserController.

Copy the store method to your new controller and delete the line $this->guard->login($user);.

It should look like this:

<?php

namespace App\Http\Controllers\Auth;

use Illuminate\Auth\Events\Registered;
use Illuminate\Http\Request;
use Laravel\Fortify\Contracts\CreatesNewUsers;
use Laravel\Fortify\Contracts\RegisterResponse;

class RegisteredUserController
    extends \Laravel\Fortify\Http\Controllers\RegisteredUserController
{

    public function store(Request $request, CreatesNewUsers $creator): RegisterResponse {
        event(new Registered($user = $creator->create($request->all())));
        return app(RegisterResponse::class);
    }

}

Finally change the default /register path that points to your new controller.

Route::post('/register', 'Auth\RegisteredUserController@store');

Fortify will automatically login an user only if you return the user from the CreateNewUser class. Instead of returning the created user, throw an exception along with a flash message. Fortify will try to redirect you to the home page and will return you back to the login page as user is not authenticated showing you the flash message. Below is a peek at the process in the file App\Actions\Fortify\CreateNewUser.

public function create(array $input)
{
    Validator::make($input, [
        'name' => ['required', 'string', 'max:255'],
        'email' => [
            'required',
            'string',
            'email',
            'max:255',
            Rule::unique(User::class),
        ],
        'password' => $this->passwordRules(),
    ])->validate();

    $user = User::create([
        'name' => $input['name'],
        'email' => $input['email'],
        'password' => Hash::make($input['password'])
    ]);

    event(new Registered($user));

    flash('Registration successful! Awaiting approval from admin.')
        ->success()
        ->important();

    throw new \Illuminate\Auth\AuthenticationException\AuthenticationException();
}

I think there might be other solution to hook into any of the Fortify events to do that even more gracefully.

go to your config/fortify.php

'features' => [
    Features::registration(),
    Features::resetPasswords(),
    //Features::emailVerification(), =>uncomment this
    Features::updateProfileInformation(),
    Features::updatePasswords(),
    Features::twoFactorAuthentication([
        'confirmPassword' => true,
    ]),
],