As mentioned by others, force_ssl_login has been deprecated and if you use it anywhere (e.g., like setting it to true in your wp-config file while debug mode mode is enabled), it may break your site, especially if you've installed and activated this plugin.
That said, if you disable all instances of force_ssl_login in the plugin's code, the plugin can still be used to turn on ssl for specific pages, as its original functionality intended, where a proper ssl certificate has been installed/configured. Fyi, I'm using it for that purpose with Gravity Forms and their Paypal Payments Pro addon.