The code in crypto/ssh/keys.go still uses insecure SHA1 signatures for RSA keys by default (unless the SignWithAlgorithm is called, but many codepaths such as SignCert will eventually call Sign which passes "" to SignWithAlgorithm. OpenSSH is going to disable this signature scheme by default in the next release as noted here.

Since OpenSSH 8.2 ssh-rsa signatures have been disabled for certificate signatures and ssh-keygen has used rsa-sha2-512 as the default for signing certificates.

I could submit a PR changing the signature function for RSA keys to be crypto.SHA512.

I also think that it's probably worth updating the verification to disallow use of SHA1 verification by default.