Still incredibly relevant. Even if you don’t apply it, there is so much to learn by reading this in 15 minutes. The only grievance I have with this is Chapter 3: Config [1] “Store config in the environment”, “Credentials to external services such as Amazon S3 or Twitter” Besides being bad advice, this had the second-order effect of leading devs to believe they could put all their local env secrets in ~/.bashrc files.…
This is actually in place in most hosting providers today - don't know if Heroku does it, but many others like Vercel and Fly will also encrypt your secret env vars and decrypt and inject them only at the last minute.
AWS itself has something similar with its secrets manager. Even in the absence of credentials, like using role based IAM when running on EC2, it probably makes sense to note that the code must access credentials by hitting a local-only metadata server - and of course this is available only when running on EC2.
For other secret like payment processor tokens, etc, there's you do need to store secrets somewhere.
Putting secrets in plaintext in the files on the execution platform is of course a problem - but that's not a problem in the 12 Factor idea - it's a security lapse in the design and architecture of the platform that is supposed to be running your 12 Factor app, if that makes sense.