Control Whether Guest Users Can See Lightning Features in Sites
Where: This change applies to Sites, accessed through Lightning Experience and Salesforce Classic in Enterprise, Performance, Unlimited, and Developer editions.
How: Test this setting in a sandbox or Developer Edition org before activating in production. To disable Lightning features for guest users, from Setup, enter Sites in the Quick Find box, then select Sites. For the site where you want to disable the setting, select Edit, and deselect Lightning Features for Guest Users.
If you apply this setting in a Lightning community, unauthenticated users can’t access any Lightning pages, including Login and Error pages. Replace these pages with custom Visualforce pages before disabling Lightning features.