From the DEPR ticket:
The django-splash repo was created 11 years ago to let the LMS redirect users to a splash screen when a user comes to the site for the first time. It works by looking for a configurable cookie value and redirecting from the middleware.
This feature was never documented, has some edx.org hardcoded defaults, and is not compatible with MFEs.
I’m not aware of anyone using this feature, and I don’t think it’s useful with our increasingly MFE-based frontends. Target acceptance date is 2024-02-09, with removal scheduled for the Redwood release.