Even using a service like Proton Mail, or using local encryption like 256 AES layered with 256 TwoFish although very secure, has inherent weaknesses. Any computer that is connected to the internet at all in any way, has weaknesses that can be attacked.
It is relatively safe as long as that connected computer is not identified as one that requires attention from the all seeing eye. Once that happens, key loggers, root kits, and even physical surveillance can overcome the best encryption and protection techniques.
And that is as it always has been. Passing information requires anonymous participants who are not and cannot be identified. Being online goes a long way to voiding that anonymity when faced with super computers and advanced AI searching for exactly those people.
True enough, that. Couriers are much more secure.