It is documentation, think of it like proof of insurance.
The electronic reader (which it will probably be, utilizing a QR code) will likely not even divulge any personal information beyond name and DOB. Just a YES or NO - if Israel, Estonia and Denmark are any indication. Your cellphone company collects and shares more sensitive information about you without your consent.
I am convinced 99% of people who say something is unconstitutional have never read the Constitution.
It only takes a few minutes.
Sounds like something the states would decide not the president.