tfrank_de, Hydration Vests. They are becoming more and more used. Not just in Gravel Races but also for Bikepacking.
I e.g. use a Camelbak Chase Vest since 2 or 3 years now (for long single day events but also for races like Badlands) and really like it.
Today Restrap added another option with it’s new Race Hydration Vest:
https://restrap.com/products/race-hydration-vest