Carrying the PSBT file throught a border and then getting the laptop checked in customs for instance.
Then don't carry it through customs. As achow has said, it is going to be publicly viewable to the entire world once you broadcast it, so you can store it on the cloud, email it to someone else or yourself, post it on a blog, any method of digital storage which you can access later once you arrive at your destination. If you do all this with a brand new and otherwise anonymous cloud/email/whatever account, then there is minimal additional risk to your privacy.
I've heard stories of people getting their laptops confiscated if they had full disk encryption and refuse to deliver a password.
If border agents are interested enough in you to force you to decrypt your entire disk, then they are going to be interested enough in individual encrypted files or containers to force you to decrypt them too. If you really want to hide something from border agents, then don't take it across the border with you. If you must, then a hidden volume with some "decoy" encrypted data is probably the way to do it, which is far outside the scope of Bitcoin Core.
I can't agree with this logic of "the entire world will know". The entire world will know X transaction happened but not that it's tied to you. The problem is linking this data to you. It's the basic pseudonymous principle of making transactions in Bitcoin. So if for instance you save a .psbt file in an usb pendrive and you forget to delete it, and someone gets access to the USB physically and manages to tie this USB pendrive to you, then they would know you are an owner of bitcoins. So you have now become a target.
But assuming this will remain as it is then you'll just have to guarantee that the file is put inside a Veracrypt container or something before it leaves your airgapped laptop.
As far as cloud storage. All of these "anonymous email" services aren't really anonymous. And "anonymous cloud services" require that you dox yourself via paying a subscription usually. You would to find one that accepts BTC and mix the coins and hope it all goes well. There's also the problem that you never know what happens with this data if you don't control the servers physically. I haven't found any reasonable way to store stuff in the cloud and call it "safe".
Carrying the PSBT file throught a border and then getting the laptop checked in customs for instance.
Wallet software is way more obvious than a PSBT. And wallet software will contain far more private information than a PSBT.
PSBTs are just base64 strings, you wouldn't know that it is Bitcoin related unless you are looking for it specifically.
And again, you can just encrypt the PSBT with a third party tool. Then it will look like an encrypted file, instead of specifically an encrypted PSBT as adding an encryption standard would make.
I have always wondered why the wallet.dat is not fully encrypted, but in any case you wouldn't move the wallet.dat file around to sign between computers as you would with the PSBT ones. Of course it's the same thing: never move the wallet.dat file around if it's not fully encrypted with a third party software.
