Okay. I have removed some of it for simplicity and yes, I've tested removing all but the home route and commenting out all the stuff I added to the boot method to no avail.

I've also tried changing the namespace of RouteServiceProvider to match the default but that resulted in the ServiceProvider not being found. I'd rather not change its namespace from what I use anyway because this is a gigantic App with a lot of classes in the 'MyApp' namespace. It's not really 'MyApp' but I've altered the name for this example.

All of this worked perfectly in Laravel 5.3 through Laravel 7

#RouteServiceProvider:

<?php

namespace MyApp\Providers;

use Carbon;
use Illuminate\Http\Request;
use Illuminate\Cache\RateLimiting\Limit;
use Illuminate\Support\Facades\RateLimiter;
use Illuminate\Support\Facades\Route;
use MyApp\Services\Help\HelpProvider;
use Illuminate\Database\Eloquent\ModelNotFoundException;
use MyApp\Services\Maintenance\Quotes\Requests\QuoteRequestProvider;
use MyApp\Services\Maintenance\Quotes\Responses\QuoteResponseProvider;
use Illuminate\Foundation\Support\Providers\RouteServiceProvider as ServiceProvider;

class RouteServiceProvider extends ServiceProvider
{
    /**
     * The path to the "home" route for your application.
     *
     * This is used by Laravel authentication to redirect users after login.
     *
     * @var string
     */
    public const HOME = '/home';

    /**
     * The controller namespace for the application.
     *
     * When present, controller route declarations will automatically be prefixed with this namespace.
     *
     * @var string|null
     */
    protected $namespace = 'MyApp\Http\Controllers';

    /**
     * Define your route model bindings, pattern filters, etc.
     *
     * @return void
     */
    public function boot()
    {
        $this->configureRateLimiting();

        $id_pattern = '^[0-9]+$';
        $guid_pattern = '^[A-F|0-9]{12}$';

        Route::pattern('quote_response', $guid_pattern);
        Route::pattern('quote_request', $guid_pattern);
        Route::pattern('reference_document', $guid_pattern);

        Route::pattern('help_doc', $id_pattern);

        Route::pattern('schedule_type', 'initial|recurring');
        Route::pattern('component_set', 'detached|attached');

        Route::model('help_doc', \HelpDocument::class, function()
        {
            throw new ModelNotFoundException(trans('resource.not_found', ['r' => trans('help.document')]));
        });

        Route::model('reference_document', \ReferenceDocument::class, function()
        {
            throw new ModelNotFoundException(trans('resource.not_found', ['r' => trans('reference.document')]));
        });

        // CUSTOM BINDINGS:
        Route::bind('help_doc_slug', function($value)
        {
            if ($help_doc = app(HelpProvider::class)->locateDocument($value))
            {
                return $help_doc;
            }

            throw new ModelNotFoundException(trans('resource.not_found', ['r' => trans('help.document')]));
        });

        Route::bind('quote_request', function($value)
        {
            if ($quote_request = app(QuoteRequestProvider::class)->findByID($value))
            {
                return $quote_request;
            }

            throw new ModelNotFoundException(trans('resource.not_found', ['r' => trans('quote.request')]));
        });

        Route::bind('quote_response', function($value)
        {
            if ($quote_response = app(QuoteResponseProvider::class)->findByID($value))
            {
                app(QuoteRequestProvider::class)->populate($quote_response->QuoteRequest);

                return $quote_response;
            }

            throw new ModelNotFoundException(trans('resource.not_found', ['r' => trans('quote.request')]));
        });

        $this->routes(function () {
            Route::prefix('api')
                ->middleware('api')
                ->namespace($this->namespace)
                ->group(base_path('routes/api.php'));

            Route::middleware('web')
                ->namespace($this->namespace)
                ->group(base_path('routes/web.php'));
        });
    }

    public function register()
    {
        //
    }

    /**
     * Configure the rate limiters for the application.
     *
     * @return void
     */
    protected function configureRateLimiting()
    {
        RateLimiter::for('api', function (Request $request) {
            return Limit::perMinute(60)->by(optional($request->user())->id ?: $request->ip());
        });
    }
}

#Web Routes:

<?php

/*
|--------------------------------------------------------------------------
| Web Routes
|--------------------------------------------------------------------------
|
| Here is where you can register web routes for your application. These
| routes are loaded by the RouteServiceProvider within a group which
| contains the "web" middleware group. Now create something great!
|
*/

// AUTHENTICATION
Route::get('login', ['as' => 'login', 'uses' => 'AuthController@getLogin', 'middleware' => 'guest']);
Route::get('auth/login', function() { return redirect('login'); });
Route::post('login', ['as' => 'login', 'uses' => 'AuthController@login']);
Route::get('logout', ['as' => 'logout', 'uses' => 'AuthController@logout', 'middleware' => 'auth']);

// HOME PAGE:
Route::get('home', ['as' => 'home', 'uses' => 'HomeController@getIndex']);

// SENDS AUTHENTICATED USERS TO THE DASHBOARD, NON-AUTHENTICATED USERS TO THE HOME PAGE:
Route::get('/', function() { return (Auth::check()) ? Redirect::route('dashboard') : Redirect::route('home'); });

// NAVIGATION:
Route::get('top-bar', ['as' => 'top-bar.get', 'uses' => 'NavigationController@getTopBar']);

// CONTACT US:
Route::get('contact', ['as' => 'contact.new', 'uses' => 'ContactController@getIndex']);
Route::post('contact', ['as' => 'contact.create', 'uses' => 'ContactController@postCreate']);

// EMAIL OPT-OUT / BLACKLISTING:
Route::get('email/optout', ['as' => 'email.optout.new', 'uses' => 'EmailController@getIndex']);
Route::post('email/optout', ['as' => 'email.optout.create', 'uses' => 'EmailController@create']);

Laravel 8 404 Everywhere and Routes Missing