<?php

namespace IronBound\WP_Notifications\Strategy;

class_alias(
  'IronBound\WP_Notifications\Strategy\Null',
  'IronBound\WP_Notifications\Strategy\Null_Mail'
);

<?php

if( version_compare( PHP_VERSION, '7.0.0' ) < 0 ) {
  class_alias(
    '\IronBound\WP_Notifications\Strategy\Null_Mail',
    '\IronBound\WP_Notifications\Strategy\Null'
  );
}

Add version check before adding alias.
Add version check before adding alias.
