I don't really get the use case.
If I reply like I do to you now, its very much meant to be a public discussion so I want people to see that so they can partake to.
There is no implication of it being private or hidden at all.
If I DM you in Amethyst I want that to be fully encrypted and metadata safe to where others can't even see we are talking, if someone can't see that they need to adopt a better client, or alternatively i'd need to explicitly full screen warning agree that I am about to leak data.
Unencrypted DM's would just be a full on no-go in that, I don't want to have to guess if my data is metadata safe, metadata unsafe or completely unsafe.
Right now Amethyst is the only Nostr app I have on Android and I like it a lot, but if I can't trust it to send DM's in a metadata safe way it means i'd instantly stop using DM's on it and i'd use a secure messenger instead.
Because to me the concept of a hidden but unencrypted message fails on a platform that lets me read them by using a pubkey.