In CRM Online, the Splash page configured on a custom object tab displays in Salesforce Classic but does not display in Lightning.
Splash page custom links are not supported in Salesforce Lightning.
- Build the Splash page as a Visualforce page.
- Create a Visualforce tab that points to the new Visualforce page.
- If tab visibility is granted at a profile level, both the custom object and Visualforce tabs display to the user.
Salesforce Help Documentation: