SHA-1 is weak: a SHA-1 collision was demonstrated and estimated to cost around $50k. https://shattered.io

Accepting SHA-1 signed certificates is a security issue, and lets attackers mount collision attacks if the CA is still signing SHA-1 certificates. crypto/x509 already rejects outright any MD5 signatures for the same reason.

The WebPKI has banned SHA-1 certificates for years now, and crypto/x509 targets a profile compatible with the WebPKI.

I propose we announce in Go 1.17 that we'll remove support in Go 1.18, and provide a GODEBUG opt-out until Go 1.19.