https://techcrunch.com/2019/01/11/shutdown-government-websites-https-certificates-expire/

In a government shutdown, everything deemed non-essential stops. As we found out, renewing the certificates on its websites is considered non-essential.

Several government sites are currently inaccessible or blocked by most browsers after their HTTPS certificate expired. With nobody available to renew them during the government shutdown, these sites are kicking back warning errors.