Pretty much everyone here is missing the point, so i guess i'll try to make it more clearly.

Once you have reasonable anticipation of future litigation, you are required to preserve evidence.

This why most litigation holds companies place do not let you delete emails, for example.

The purpose of discovery is to get all relevant evidence out in the open in order to help resolve the case.

These two things are true regardless of medium.
IE If i am making phone calls that are relevant to litigation, and you otherwise would keep records or recordings of them, you must preserve them. I can be called to testify to them.

Use of mediums like this is a grey area, but that's irrelevant. You can argue till you are blue in the face that you have legitimate reasons to use self-destructing/encrypting mediums.

"Once a party has notice that litigation has been filed, courts uniformly impose a duty to preserve potentially relevant evidence on parties to the lawsuit. The duty “includes an obligation to identify, locate, and
maintain, information that is relevant to specific, predictable, and identifiable litigation.” The duty applies only to relevant data, documents and things."

Remember that at one point, email preservation and other things did not exist.
Courts forced companies to preserve that data:
"To be sure, as part of a litigation hold, a company may be required to cease deleting e-mails, and to disrupt its normal document destruction protocol."

So let's turn to here.
First, regardless of anything else, the use of anything with a specific goal of avoiding the duty to preserve evidence is going to be held against you.
Full stop. It does not matter what that is.

So regardless of the medium in use here, that's problem #1.
How far does anticipation of a lawsuit go?

In the oft-cited Zubalake decision, the court found that a company employer had a duty to preserve electronic records destroyed before an employee filed the charge of discrimination that triggered a government investigation because almost everyone with whom that employee worked anticipated she might bring a lawsuit. That is, the court held that duty to preserve attached at the time that litigation was “reasonably anticipated,” and that key company employees anticipated litigation months before the employee filed a charge of discrimination.

Boom. This case is pretty much already lost in those jurisdictions.

Problem #2 is what if the medium normally keeps no records.
Again, if you did it deliberately to avoid discoverability, you are already going to lose.

It's true that there is no general duty to preserve, but once that duty kicks in, the fact that the medium is ephemeral is irrelevant.
There is literally nothing that prevents preservation of evidence here other than desire. They could make records of conversations (screen grabs, what have you), they could record the ephemeral keys and data (they have physical access), and they could also tell people who they anticipate (above) to be involved in litigation to not use such mediums, and in fact, pretty easily force compliance.

I'm aware of zero cases that have said "yeah, it sucks that you lost those ephemeral messages", and plenty that have sanctioned as a result of failure to save.
See all the linked cases in https://www.lexology.com/library/detail.aspx?g=f47f71d2-281b...

You should also realize that if these forms of communication become incredibly common, the discovery rules will adapt.

They have adapted as texting and instant messaging became more common, they will adapt as self-destructing messages become more common. That adaptation is not going to be "self-destructing messages get a free pass", it's going to be "you may be required to preserve keys"