In general you don't want governments or just anyone to know you are transacting in Bitcoin and PSBT shows you are in.
PSBT file is relative small, you could always hide it inside regular file (e.g. png or docx).
I really love this idea! I forgot about it since I haven't heard about it in a while, but yeah, I remember, it's quite easily possible to store whole papers in jpegs with minimal modification of the look of the image when opened normally.
Here's an online tool to try
Steganography.
From
https://en.wikipedia.org/wiki/Steganography, very impressive:

That's correct, I tried to create a Protonmail account via Tor Browser a few times already and compared to accessing the site over clearnet, it asks for verification using Email, SMS or payment.