Hardening SSH algorithms, which typically means dropping
all-but-the-strongest is of questionable value, given SSH's downgrade
protection[0]. We pay in compatibility, and maintenance.

Further, as noted in
https://github.com/NixOS/nixpkgs/pull/172393/files#r871727289 , both
the guidelines that we follow have not been updated in years.

The costs of having/maintaining these defaults:

* The burden of having a larger module that deviates from
upstream. We've slowly been reducing the upstream diff, to reduce
maintenance burden.
* Difficult for users to opt-out of these defaults. For example, when
using a "no OpenSSL" build of OpenSSH, having these defaults means
having to manually overriding NixOS's defaults. Upstream's defaults,
meanwhile, gracefully only use available algorithms, if OpenSSL is not
linked.
    * For users seeking to reduce attack surfaces that are fortunate
    enough to only have modern clients, they could choose to use
    `pkgs.opensshPackages.openssh.override { linkOpenssl = false; }`,
    which only supports chacha20-poly1305 and curve25519-sha256.
* NixOS#231165 unexpectedly broke some
clients.
* The time in discussing/reviewing these defaults.
* Anecdotally, a friend trying NixOS for the first time with a
ssh_config supporting only ecdh-* key exchanges was unable to SSH in
after install.

There's a certain level of enjoyment that comes from researching and
selecting a favourite suite of ciphers, but as a distro, it's not our
core competancy, and best left for upstream who are active in
advances/attacks/compatibility.

0. https://eprint.iacr.org/2016/072.pdf