Further, the best thing you can to protect your assets from a crash is: have a regular investment plan, follow it, and stop looking at the market.
If you’re ten or more years from retirement, the overwhelming likelihood is that a crash simply does not matter. A drop in prices only matters when you’re buying (good) or selling (bad). Everything in between is noise that only affects you on paper. Selling due to a crash is the ideal way to lock in losses. Just contribute to your retirement accounts as normal, regardless of what the market is doing. If anything, feel happy you’re getting a discount on your regular purchases.
If you’re nearing retirement, the answer is that you should be gradually shifting over time from riskier assets to more conservative ones. While young you might be at and 80:20 ratio of stock market index funds to bond market index funds, but nearing retirement that might flip to 60:40 with the remaining 10% in fixed-income assets (exact numbers are determined by your personal risk threshold and not meant as absolutes). For most people, a Target Retirement fund is probably the best choice here since they’re fire-and-forget.
As an anecdote, I have a friend who felt very strongly that a crash was imminent in early 2018. He argued passionately that markets would drop, and wouldn’t it be great to be in a position to be able to swoop back in and grab everything at a discount when this happened? I bet beers that the S&P would be greater than or equal to its current price (~2715) on Jan 1 2019. He also sold everything and held cash.
Well, as luck would have it, there was a “crash”. Based on the exact timing of our bet, I ended up losing by a few dollars (it was at ~2700) and had to buy him beers. He stayed in cash while I’ve continued buying. If he’d had perfect knowledge and changed course at rock bottom, he would have squeezed out an extra 8% return. In practice, he’s actually lost out on a 40%+ return that required simply doing nothing. I think my strategy has pretty soundly won in retrospect.