Local Php Issue possibly related to Mac Upgrade?

Even though your EE2 installs (that are 2.11.9) should be PHP 7.1 compatible, that doesn't mean that your add-on's are. I know @JCOGS recommended renaming your third_party as a way to test, but I don't believe that is the best method for testing a base install with all third party software disabled.

Make sure that you have debug in your index.php set to 1. Next, in your config.php file, turn extensions off:

https://docs.expressionengine.com/v2/general/system_configuration_overrides.html#allow-extensions

$config['allow_extensions'] = 'n';

Now, I would recommend renaming your third_party folder, but I would also recommend backing up your exp_modules table, and then truncating it. I believe EE2 also had an exp_accesories table as well, do the same there. All of these things combined should set your EE install to actually believe their are no modules, extension, plugins, or accessories installed.

Note that if you are still receiving WSOD, you have an issue with error reporting. Regardless of other issues, unless you find something in the Apache/PHP error logs, you should be receiving an error of some sort.

It depends what version of EE2 you are running - only the final few versions of EE2 support php7. The most compatible version (tested for php7.1) is the final version EE2.11.9.

You may also need to update the add-ons to work with php7: what is required will depend on what add-ons and whether they were still being supported when the transition from php5 to php7 took place.

The WSOD is one of the most annoying aspects of the way EE responds to such issues, and there are some fairly well developed work-arounds - for example simply renaming your third-party add-ons folder to something else to prevent EE from trying to load them so you can work out the issues with getting EE itself working with php7, and then add the add-ons back one at a time so you can work out which ones are causing the WSOD.

HTH