We use them in my current company. I've found them useful and they have great buy in across the engineering org. There does need to be a certain level of commitment to make it worthwhile, feedback needs to be taken onboard and processes and reward need to align to the principles also.

Engineering Principles: https://github.com/Skyscanner/engineering-principles Some background (may be slightly out of date): https://medium.com/@SkyscannerEng/an-update-on-our-engineeri...

In summary - definitely useful, but to avoid the issue you raised around documentation like this becoming a relic you need to commit as an organisation to living by the principles and reviewing them often enough

(*all my opinions - not the necessarily the opinions of my employer)