amalyshev is correct. But to clarify:
sudo nano /etc/ssh/ssh_config
Find the line:
Ciphers aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blow fish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour
Insert a new line directly underneath and paste this line in:
Save the file and exit. ssh should work as usual.
This isn't so much an Apple thing as it is an OpenSSH thing. High Sierra's openssh has been updated to OpenSSH_7.6p1, which deprecates a bunch of older configs. Release notes are here. But basically they have removed support for the arcfour, blowfish and CAST ciphers. Older systems that are running old OpenSSH servers will still present old ciphers (eg my work NAS only supports up to aes256-cbc) so will need these as options.
If you follow amalyshev's instructions to the letter and simply uncomment the line you'll get an error similar to:
/etc/ssh/ssh_config line 33: Bad SSH2 cipher spec 'aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blo wfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour'.
This is because the arcfour, cast and blowfish ciphers are no longer supported.