Rationale: NPN has been superseded by ALPN. Chrome dropped support a year ago in Chrome 51. It's not needed for HTTP/2.

Strawman proposal:

• deprecate tls.convertNPNProtocols() in node 9 (or now - it's undocumented)
• warn on NPNProtocols option or access of .npnProtocol property in node 9?
• remove in node 10

Backwards compatibility concerns:

• OpenSSL < 1.0.2 supports NPN but not ALPN. Doesn't affect Node.js itself but there might be interoperability issues with TLS-based services that are linked to obsolete versions of OpenSSL.

Chrome already paved the way, though. I don't see it as a serious concern.

cc @nodejs/security