The DiffieHellmanGroup14SHA1 ("diffie-hellman-group14-sha1") cipher is available on all platforms and is used when it's enabled by SshParameters.KeyExchangeAlgorithms and has not been disabled using SshParameters.SetKeyExchangeAlgorithms(...) method.
However, please note that this cipher is now considered week, because it uses a relatively weak Diffie-Hellman group and SHA-1 hash algorithm. It will most likely be disabled by default in a year or two.