sshd: Service exited with abnormal code: 255

In recent macOS versions, the rotten fruit geniuses have restricted SSH access to admin users only. I was having this 'service exited with abnormal code' issue and I fixed it by Preferences -> Sharing -> Remote Login: On and 'Allow Access to': All Users (or the users/groups you want). You must be an admin to do that (you must be able to unlock the locker icon on the bottom left).

Two hours wasted, damn idiots...

I was running into this issue because of the default "Allow ssh access only to admin" setting, but wanted a command-line only solution.

I changed it by adding the user to the admin (and wheel for good measure) group:

sudo dscl localhost -append /Local/Default/Groups/admin GroupMembership $USER
sudo dscl localhost -append /Local/Default/Groups/wheel GroupMembership $USER