x/crypto/ssh has "arcfour256", "arcfour128" in its list of default ciphers. These ciphers are now considered weak. I seems like it might be time to disable these by default as the documentation says:

// The allowed cipher algorithms. If unspecified then a sensible
// default is used.
Ciphers []string

These two ciphers are rc4 variants. They do discard the first 1536 bytes of the cipher stream so they are better than plain rc4 but they are still considered weak.

OpenSSH disabled them by default in 7.2 (2016-02-29) and removed them completely in release 7.6 (2017-10-03).

RC4 was disabled by default in crypto/tls in 2015: #10094.

I was dinged in a security audit because these ciphers were enabled. I can obviously remove them from my config, but with the OpenSSH changes it seemed like it might be more appropriate to update the defaults.

If this change is acceptable I'm happy to something for code review.