Is the latest Drupal 7 core, 7.59, fully PHP 7 ready?
For the most part, Drupal 7 core is compatible with PHP 7, however in some cases you might run into issues, depending on your specific website's configuration.

There is a meta issue on drupal.org that you can follow at https://www.drupal.org/project/drupal/issues/2656548

In addition, although most of the widely used modules are, not all contributed modules are compatible yet.
The issue you linked is for a contributed module, not Drupal core.

Your best bet is to test your site and its particular modules on a development PHP 7 environment and then resolve any issues you find. Once you're happy that the site is functioning as it should, update your PHP version on your production server.

In my personal experience, with at least a dozen Drupal 7 sites of varying sizes, I have had no issues with Drupal 7 on PHP 7.0 or 7.1 but I have had a few issues with 7.2 when it was new (which may have been resolved by now).

In some cases I have had to apply patches that were already available in issue queues, but generally I've only had to update modules to the latest versions.