In June 2018, in an effort to raise security and comply with modern standards, the insecure TLS 1.0 & 1.1 protocols will no longer be supported for SSL connections to Central. This should only affect users of Java 6 (and Java 7) that are also using https to access central, which by our metrics is less than .2% of users.
For more details and workarounds, see the blog and faq here: https://blog.sonatype.com/enhancing-ssl-security-and-http/2-support-for-central