yep, but not only decentralized, I remember the issue about a company claiming they could extract Signal messages from the phone, if they were able to overcome the phone encryption (easily if the phone is unencrypted, though I believe LOS AOSP in general don't allow unencrypted phones fro quite some time now)...

I'm wondering if p2p app also suffer the same. I honestly have no clue, but I would guess it should be the same, unless you provide a decrypting password or mechanism any time you open the app, to decrypt keys keeping the contents encryupted (supposing it keeps some sort of agent when opening, so you don't need to keep decrypting the keys while the app is open)... Perhaps they don't do it by default, but AFAIK, briar, jami, antox and triffa were not having content encrypted in the phone, and I'm not sure if there was an option to opt-in for such encryption...

Going back to xmpp, the same mentioned on the post, applies to the desktop apps, for example, I'm not awae of Dino, Gajim or Kaidan encrypting local content... I'm wondering if there are apps doing that by default, and perhaps without opt-out mechanisms...

That's why both, the phones and the desktops/laptops/mobile/etc, require to have the storage encrypted. And even though the storage is encrypted, all private keys (GPG and/or SSH) must be encrypted as well. There are people keeping private keys unencrypted, for convenience I'd guess, but that's really bad if the device keeping them is compromised...

In brief, I'm not aware of apps, phone or desktop, which besides doing e2ee for content transmission, they keep all local content encrypted. Not sure if there are some which can do it by setting some preferences, but if there are, please share which ones, :) Thanks !