I don't see how that is relevant? You're already familiar with C, so writing about C does not influence whether you will be outdated in a few years. Learning and writing about Rust could be something that becomes useful, but not necessarily practically - it depends on what you will do in the future.

If you feel you lost your passion, I would suggest learning and experimenting with Rust. It's different, so may be interesting and thought provoking to learn.

Writing down what you know about C may also be worthwhile, good, or produce a good resource, but I don't see it as much or like as sparking lost interest and passion. If that is actually your goal (you only asked about future relevancy in the end).