How can I roll back?
If you don’t have the anonymous REST API issue, rolling back may not help you. You may have some other plugin or theme conflict.
The CF 7 4.8 -> 4.7 rollback is easy since there don’t appear to be any database changes. As with any plugin, you go to the plugin home page, click the “Advanced View” link in the right sidebar, scroll down to “Previous Versions,” and download the version you want to use.
Then, it’s a matter of deleting the files for the current version of the plugin and uploading the version you want to use. If you have a hosting control panel like cPanel, the fastest way I’ve found is to upload the entire zip file for the version you want to roll back to to your /wp-content/plugins folder, use the File Manager of your control panel to delete the /wp-content/plugins/contact-form-7 folder, extract the zip file, and then clear your cache if you’re using some form of caching. There are also plugins that will roll back versions.