Remove old user from Remote Login permissions list
Found the solution here, hope this helps.

1) First remove all users that can be removed from GUI In System Preferences -> Sharing -> Remote login

2) Run the below command which shows the configuration of users who have remote login access

sudo dscl  . -read /Groups/com.apple.access_ssh


example result:

AppleMetaNodeLocation: /Local/Default
GeneratedUID: 2F60778C-0931-47A4-B656-DBE07ACD843F
GroupMembers: **3D7C31B1-DEDE-4A5A-AAA8-CDD55645DC75**
GroupMembership: youraccoutname
PrimaryGroupID: 404
RealName:
Remote Login ACL
RecordName: com.apple.access_ssh
RecordType: dsRecTypeStandard:Groups


3) remove the group GroupMembers that the user(Unknown User) is in

sudo dscl  . delete /Groups/com.apple.access_ssh GroupMembers 3D7C31B1-DEDE-4A5A-AAA8-CDD55645DC75


4) Go to System preferences and see the result