ClassicPress v2.0 is a rework of WordPress 6.2.x. With that we dropped the Security page as it never really gained developer traction.
If your theme is going to support WordPress and ClassicPress then the Bast Practice section is pretty important if you are going to use functions that are not available in ClassicPress. (You can open an Issue requesting a back port is done too). The Providing Shims section might be helpful to develop cross platform support too.
Beyond that, develop your theme locally with WP_DEBUG set to true in your wp-config.php file so that you can easily see and fix warnings and error as you develop.