Add arcfour Cipher to Mojave 10.14
Support for Arcfour was disabled by default in OpenSSH 7.2 (Feb 2016), and was subsequently removed completely in OpenSSH 7.6 (Oct 2017). macOS 10.14 ships with OpenSSH 7.9, which is several major versions beyond this removal.

In theory, you could reverse some of the changes applied in OpenSSH 7.6 to readd support for Arcfour. However, doing so will require significant C programming experience.

In practice, I would advise you to reconsider what you are doing.