Some suggestions: https://blog.1password.com/where-to-store-your-emergency-kit...
Even if you write it down on a post-it note, you now need a physical attack to access it on top of whatever attack you needed for the master password.
The key is 38 characters long, which is certainly within the range of human memory, if you're inclined to memorize it.
It‘s a strictly additional barrier to the passphrase. Bitwarden and LastPass don‘t use one, so if you are fine with that security level, you could publish it on your blog, GitHub profile, or Wikipedia talk page and would literally not be less secure than those two.
It really only is defense in depth, but in a breach like this is where it could shine. A parallel brute force attack becomes infeasible if attackers need to also compromise cloud or local storage, or even trivially correlate semi-publicly but unstructuredly posted secret keys to accounts/vaults.
Thanks, folks, I should have jsut RTFA'd I misunderstood secret key as a failsafe, it is not:
https://support.1password.com/secret-key-security/
I installed both LastPass and 1Password and they make my homegrown key storage system feel so awkward by comparison.