<?php
if (!class_exists('Cake\Http\Exception\MethodNotAllowedException')) {
    class_alias(
        'Cake\Network\Exception\MethodNotAllowedException',
        'Cake\Http\Exception\MethodNotAllowedException'
    );
}

if (!class_exists('Cake\Http\Exception\NotImplementedException')) {
    class_alias(
        'Cake\Network\Exception\NotImplementedException',
        'Cake\Http\Exception\NotImplementedException'
    );
}

if (!class_exists('Cake\Http\Exception\BadRequestException')) {
    class_alias(
        'Cake\Network\Exception\BadRequestException',
        'Cake\Http\Exception\BadRequestException'
    );
}

if (!class_exists('Cake\Http\Exception\NotFoundException')) {
    class_alias(
        'Cake\Network\Exception\NotFoundException',
        'Cake\Http\Exception\NotFoundException'
    );
}

<?php
if (!class_exists('Cake\Http\Exception\MethodNotAllowedException')) {
    class_alias(
        'Cake\Network\Exception\MethodNotAllowedException',
        'Cake\Http\Exception\MethodNotAllowedException'
    );
    class_alias(
        'Cake\Network\Exception\NotImplementedException',
        'Cake\Http\Exception\NotImplementedException'
    );
    class_alias(
        'Cake\Network\Exception\BadRequestException',
        'Cake\Http\Exception\BadRequestException'
    );
    class_alias(
        'Cake\Network\Exception\NotFoundException',
        'Cake\Http\Exception\NotFoundException'
    );
}

--- 
+++ 
@@ -4,14 +4,23 @@
         'Cake\Network\Exception\MethodNotAllowedException',
         'Cake\Http\Exception\MethodNotAllowedException'
     );
+}
+
+if (!class_exists('Cake\Http\Exception\NotImplementedException')) {
     class_alias(
         'Cake\Network\Exception\NotImplementedException',
         'Cake\Http\Exception\NotImplementedException'
     );
+}
+
+if (!class_exists('Cake\Http\Exception\BadRequestException')) {
     class_alias(
         'Cake\Network\Exception\BadRequestException',
         'Cake\Http\Exception\BadRequestException'
     );
+}
+
+if (!class_exists('Cake\Http\Exception\NotFoundException')) {
     class_alias(
         'Cake\Network\Exception\NotFoundException',
         'Cake\Http\Exception\NotFoundException'
Check for each class individually.

One of more could have been already aliased by other plugins like ADmad/Glide.