Even KeePass recommends to write the master secret key on a piece of paper. Sure, it must be in a secure place, but we cannot avoid this step for reliability.

No it’s not

An A-Z character string written inside random page a book stored at a relative's house is a super easy way to secure a physical backup of your private key. You could even get clever and encrypt the key using some memorable passage of another book. Since the book passage is unknown to anyone else and be selected from a near infinite pool of paragraphs from the entirety of all published books, it would be impossible for someone to bruteforce even if they found the book where you stored the encrypted key. This would be an easy enough process to generate the ciphertext and store it in the event that you totally lost access to all of your electronics, online accounts, and home.

You are not adding any more security than that if you simply remembering some random key (which isn’t that hard if you put your mind to it).

Don't forget the I and the A in the CIA triad, which are likely to be improved by keeping things recorded in writing.

Sometimes it is: https://en.bitcoin.it/wiki/Cold_storage

You could just save it on a flash drive and hide it in safe. How does printing it on a piece of paper make it any safer?

With paper you can be confident that you will still be able to read it in decades. How true is that of a flash drive?

Did you even bother viewing the linked page?

> The advantage is that any security issues of USB interfaces or cameras are completely avoided.

There's also the section that enumerates all the downsides for each medium of storage: https://en.bitcoin.it/wiki/Cold_storage#Private_key_backup_s...