I have some legacy devices in my network that require older SSH key exchange methods and algorithms incompatible with modern defaults. Rather than configuring each host individually in ~/.ssh/config, I modified the system-wide SSH client configuration to maintain compatibility with both old and new systems.

• nano /etc/ssh/ssh_config

HostKeyAlgorithms ssh-dss,ssh-rsa,ssh-ed25519,ecdsa-sha2-nistp256,rsa-sha2-512,rsa-sha2-256
PubkeyAcceptedAlgorithms ssh-dss,ssh-rsa,ssh-ed25519,ecdsa-sha2-nistp256,rsa-sha2-512,rsa-sha2-256
KexAlgorithms diffie-hellman-group1-sha1,curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256
Ciphers aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc
MACs hmac-md5,hmac-sha1,umac-64@openssh.com