It's not really a travel warning. It's an addition to the info about what's needed to enter the US.
You now need a passport that specifies "male" or "female" as your gender.
What if a country decided to completely remove the gender information from their passports? The US would then have to decide whether to ban everyone from that country or allow everyone?
I would think the only reason for gender in a passport is to help identify, as like big guy with beard on the picture would have male. Which seems incomparable with the more modern gender identity concepts