Just noticed this today on my 1040V. Thought it might an April Fool's joke by some prankster at IRS but apparently the online form shows it too.
https://www.irs.gov/pub/irs-pdf/f1040v.pdf
"No checks of $100 million or more accepted. The IRS
can’t accept a single check (including a cashier’s check)
for amounts of $100,000,000 ($100 million) or more. If you
are sending $100 million or more by check, you will need
to spread the payments over two or more checks, with
each check made out for an amount less than $100 million."
https://www.irs.gov/pub/irs-pdf/f1040v.pdf
"No checks of $100 million or more accepted. The IRS
can’t accept a single check (including a cashier’s check)
for amounts of $100,000,000 ($100 million) or more. If you
are sending $100 million or more by check, you will need
to spread the payments over two or more checks, with
each check made out for an amount less than $100 million."