As of December 2019, PHP 5.6, 7.0, and 7.1 have been removed from the DreamHost panel.
If you would like to manually change the PHP version of your site, the current options are 7.4, 8.0, and 8.1. For more information, see the following article:
What if my site was previously using PHP 5.6, 7.0, or 7.1?
Any site on a shared server previously running PHP 5.6, 7.0, or 7.1 was automatically upgraded to PHP 7.2.
It's highly recommended that you upgrade your sites to PHP 7.4, 8.0, or 8.1 as older versions will soon be removed completely from all DreamHost servers.
For more information on updating your site's PHP version, view the following article:
Can I continue to use an older PHP version?
Yes. DreamHost offers an Extended Support plan which allows you to use outdated versions of PHP. View the following article for further details.