""I'm curious whether there are good prospects for 'time capsule encryption,' one of several ways of storing information that renders it inaccessible to anyone until certain conditions â" such as the passage of time â" are met?"
The motivation for this question is vague. It could be that the OP has information about a criminal element that she wants released if she suffers an untimely death. It could be that the OP has solved the problem of nuclear fusion but is not ready to share it yet. The motivation is so vague that there is no way to address the question coherently - let's assume it's just for releasing info at a much later time.
'Time capsule' - I attended a time capsule burial a while back. Someone will dig it up in 100 years. It contains a variety of stuff- printed text, objects & some digital material. The digital stuff will probably be indecipherable with equipment available in the year 2108. The 'time capsule' concept might still be best despite our gravitation to digital and the 'cloud'. Encryption will not be necessary.
Printed text on quality paper should be good for well over 100 years. Physical materials might be the best way to preserve the message. A physical location might be the best place. A simple timer that sets off a weak explosion that exposes the trove might be ideal. Locate the capsule thoughtfully- not in downtown London, not in Antarctica, not in the Mariana Trench. Protect the payload from the elements. The timer & explosives need to survive the time you set. You might offer hints to potentially interested parties about the locale and timing of the release of your important capsule.
But before you go to all this trouble you should ask yourself- what information do you have that might matter to people in the future? Is this just an ego stunt or something that might really benefit someone in that time?