the page https://dlang.org/download.html contains the archive link http://downloads.dlang.org/ which is not secure. All links in there, secpecially http://downloads.dlang.org/releases/2.x/, are not secure either, including signatures.
By secure you mean confidential? Authenticity of asymmetric signatures relies on secrecy of signer's private key, not on confidentiality of signature itself.
cmon ;) you know what I mean. https.