I hear this over and over. Don’t experiment with new clothing, nutrition, gear, etc on race day. As a general principle it makes sense, but one must also account for real life.
I train in a cold environment and races tend to be in very hot environments. I ran in snow yesterday. In less than two weeks I will be racing, most likely, in mid-80s and humid.
I could make attempts to better emulate race conditions. I could run on an indoor treadmill in a warm room without a fan. I could keep the fan off on my bike and turn up the heat. I could do exercises in the sauna. But at this stage, I am willing to do none of these things. I had a lovely, enjoyable run getting outside with the dogs. Indoors would have been quite miserable without tired doggies afterward.
Thus, plenty will be new for me on race day. After all each race is different. Different terrain, different weather. Even the same course will be different every year based on different conditions. Furthermore, with an iron-distance race, no one I know trains at the full distances for anything, much less putting them all together, so race day is a whole different story than training days.
Try out what you can in terms of gear and nutrition, but understand both necessary parameters and don’t go so far that it takes the fun out of the sport.