Somewhere on your server there is a 94-page PDF. It cost a lot. It is beautiful. It was presented to the business at a launch event with sandwiches, and three people have opened it since.
This is such a common outcome that it has stopped being treated as a failure. It is treated as weather.
It is a failure, and it is a failure of format rather than of thinking. The thinking in most of those documents is fine. The format is designed for the moment of presentation, not for the moment of use, and those two moments have almost nothing in common.
Presentation wants a narrative arc, context, rationale and a reveal. Use wants an answer in under thirty seconds, at 4pm, from someone who is not a brand specialist and has a thing to ship. A document optimised for the first is close to useless for the second, and since the second happens ten thousand times more often, the document is useless almost all of the time.
Here is what actually gets used, in my experience of both making these and failing to follow them.
Decisions, not principles. "Be confident but approachable" is not usable, because two people will read it in opposite directions and both will be right. "We use contractions. We do not use exclamation marks. We never describe ourselves as passionate." Those are usable, because they can be complied with or broken, and you can tell which has happened.
Examples over abstraction, and bad examples especially. The single most useful page in any guideline is a side-by-side of a right version and a wrong version, with one line on why. Most guidelines contain many right examples and no wrong ones, which is like teaching someone a language using only correct sentences and never pointing at a mistake.
Answers to the questions that actually get asked. Before writing anything, go and find the last fifty brand questions people asked internally. It will be a short and repetitive list: what do I do about the logo on this, can I say this, which of these two fonts, is this tone right for a complaint. Answer those first. That is the document.
Searchable and in the workflow. A PDF in a shared drive is functionally hidden. It needs to be a web page, or in the tools people already work in, and it needs to survive being searched for one word at speed.
An owner and a change log. Guidelines that never change are guidelines nobody is using, since real use generates edge cases and edge cases generate updates. Silence is not compliance. It is abandonment.
The deeper point is that a brand system is not a publication. It is infrastructure, and infrastructure is judged on whether people can operate it without training. If your last one required a launch event to explain, that was the tell.