Is the latest Drupal 7 core, 7.59, fully PHP 7 ready? I just transferred a Drupal 7 installation to Debian 9 and when I try to connect to the site I get the error PHP Fatal error:  Cannot use "self" when no class scope is active in vars.module on line 39. 

There is a Drupal issue on it Upgrade to PHP7: Cannot use "self" when no class scope is active [#2668622] , but it doesn't seem to be fully resolved and I am not sure which PHP7 version it works on.

The version of PHP currently on Debian 9 is PHP Version 7.0.27-0+deb9u1

The point is I am running the latest version (Drupal 7.59) on Ubuntu 16.04 whose PHP version is PHP Version 7.0.30-0ubuntu0.16.04.1 and it runs without that error.

Are there versions of PHP 7 running on Debian 9 which work with latest unpatched Drupal 7.59, perhaps later ones like PHP 7.1 or PHP 7.2?

Maybe the bug only manifests in  early versions of PHP 7.

UPDATE: It turns out that vars.module is from the contrib module Variable API and not part of core, and was only enabled to support the Textile module. Disabling it solved the problem.
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.