For disappearing messages to work, your conversation partner has to promise they won’t take photos of their screen, and they have to promise to use an app that actually implements the feature instead of just pretending to, and the app developers have to promise to have implemented the code to delete a message when the service says it should
Is there actually a cryptographically-sound and physically-complete method for ensuring that a message is only legible for a temporary duration once it leaves your own device and is delivered to someone elses?
One example I can think of is Widevine DRM, which is owned by Google and is closed source: https://en.wikipedia.org/wiki/Widevine
Google currently allows Mozilla (and others) to distribute this within Firefox, allowing Netflix, Disney+, and various other video streaming services to work within Firefox without any technical work performed by the user
I don’t believe Google would ever willingly take this away from Mozilla, but it’s entirely possible that the movie and music industries pressure Google to reduce access to Widevine (the same way they pressured Netflix into adopting DRM)