Whether to enable or not the old default config values. This option will become deprecated in the future. For an equivalent, copy and paste the following code snippet in your config:
programs.ssh.matchBlocks."*" = { forwardAgent = false; addKeysToAgent = "no"; compression = false; serverAliveInterval = 0; serverAliveCountMax = 3; hashKnownHosts = false; userKnownHostsFile = "~/.ssh/known_hosts"; controlMaster = "no"; controlPath = "~/.ssh/master-%r@%n:%p"; controlPersist = "no"; };