The checkout page contain standard fields , in many cases the site owner needs to hide some fields for specific reason , the system give you the ability to hide some fields directy from xml , you can add checkout_index_index.xml (copy same structure or just what you want from this path vendor/magento/module-checkout/view/frontend/layout/checkout_index_index.xml) to you theme or your custom extension and start hiding some fields .

Hide Zip Code :
Go to file and search for postcode and add or override tag :

<item name="visible" xsi:type="boolean">false</item>

<item name="postcode" xsi:type="array">
   <item name="visible" xsi:type="boolean">false</item>
        <item name="component" xsi:type="string">Magento_Ui/js/form/element/post-code</item>
              <item name="validation" xsi:type="array">
                  <item name="required-entry" xsi:type="string">true</item>
               </item>
   </item>

<item name="company" xsi:type="array">
     <item name="visible" xsi:type="boolean">false</item>  
            <item name="validation" xsi:type="array">
               <item name="min_text_length" xsi:type="number">0</item>
            </item>
</item>

The same case for some fields (and the data will saved in db without backend validation) , but for  city , region , country , street , firstname and alst name it's have specific backend validation with php …..

Hide City Case :
First the same thing in checkout_index_index.xml :

<item name="city" xsi:type="array">
       <item name="visible" xsi:type="boolean">false</item>
</item>

public function validate()
    {
        $errors = [];
        if (!\Zend_Validate::is($this->getFirstname(), 'NotEmpty')) {
            $errors[] = __('Please enter the first name.');
        }
        if (!\Zend_Validate::is($this->getLastname(), 'NotEmpty')) {
            $errors[] = __('Please enter the last name.');
        }
        if (!\Zend_Validate::is($this->getStreetLine(1), 'NotEmpty')) {
            $errors[] = __('Please enter the street.');
        }
        if (!\Zend_Validate::is($this->getCity(), 'NotEmpty')) {
           $errors[] = __('Please enter the city.');
        }
         if (!\Zend_Validate::is($this->getTelephone(), 'NotEmpty')) {
            $errors[] = __('Please enter the phone number.');
        }
        $_havingOptionalZip = $this->_directoryData->getCountriesWithOptionalZip();
        if (!in_array(
            $this->getCountryId(),
            $_havingOptionalZip
        ) && !\Zend_Validate::is(
            $this->getPostcode(),
            'NotEmpty'
        )
        ) {
            $errors[] = __('Please enter the zip/postal code.');
        }
        if (!\Zend_Validate::is($this->getCountryId(), 'NotEmpty')) {
            $errors[] = __('Please enter the country.');
        }
        if ($this->getCountryModel()->getRegionCollection()->getSize() && !\Zend_Validate::is(
            $this->getRegionId(),
            'NotEmpty'
        ) && $this->_directoryData->isRegionRequired(
            $this->getCountryId()
        )
        ) {
            $errors[] = __('Please enter the state/province.');
        }
        if (empty($errors) || $this->getShouldIgnoreValidation()) {
            return true;
        }
        return $errors;
    }

Let's start  override the function . first create the di,xml inisde you extension directly in folder etc YourVendor/YourExtName/etc/di.xml  and fill :

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Magento\Quote\Model\Quote\Address" type="YourVendor\YourExtName\Model\Quote\Address" />
</config>

Now create the class  Address inside YourVendor/YourExtName/Model/Qoute/Adress,php and push :

<?php
namespace YourVendor\YourExtName\Model\Quote;
class Address extends \Magento\Quote\Model\Quote\Address
{
   /**
     * Validate address attribute values
     *
     * @return bool|array
     * @SuppressWarnings(PHPMD.CyclomaticComplexity)
     * @SuppressWarnings(PHPMD.NPathComplexity)
     */
    public function validate()
    {
        $errors = [];
        if (!\Zend_Validate::is($this->getFirstname(), 'NotEmpty')) {
            $errors[] = __('Please enter the first name.');
        }
       if (!\Zend_Validate::is($this->getLastname(), 'NotEmpty')) {
            $errors[] = __('Please enter the last name.');
        }
        if (!\Zend_Validate::is($this->getStreetLine(1), 'NotEmpty')) {
            $errors[] = __('Please enter the street.');
        }
         if (!\Zend_Validate::is($this->getCity(), 'NotEmpty')) {
            //$errors[] = __('Please enter the city.');
        }
        if (!\Zend_Validate::is($this->getTelephone(), 'NotEmpty')) {
            $errors[] = __('Please enter the phone number.');
        }
        $_havingOptionalZip = $this->_directoryData->getCountriesWithOptionalZip();
        if (!in_array(
            $this->getCountryId(),
            $_havingOptionalZip
        ) && !\Zend_Validate::is(
            $this->getPostcode(),
            'NotEmpty'
        )
        ) {
            $errors[] = __('Please enter the zip/postal code.');
        }
         if (!\Zend_Validate::is($this->getCountryId(), 'NotEmpty')) {
            $errors[] = __('Please enter the country.');
        }
        if ($this->getCountryModel()->getRegionCollection()->getSize() && !\Zend_Validate::is(
            $this->getRegionId(),
            'NotEmpty'
        ) && $this->_directoryData->isRegionRequired(
            $this->getCountryId()
        )
        ) {
            $errors[] = __('Please enter the state/province.');
        }
       if (empty($errors) || $this->getShouldIgnoreValidation()) {
            return true;
        }
        return $errors;
      }
}

In Our Example we have commented the code related to city test : 
       

 if (!\Zend_Validate::is($this->getCity(), 'NotEmpty')) {
            //$errors[] = __('Please enter the city.');
        }

Finally if you test now to submit the order , you will have the success page without errors and system skip the backend validation of city …..