The user handling in openshift is handled in different kinds. I Use here the LDAP Identity Providers as example.
As the official documentation describes you can use different back ends to authenticate a user.
When a user is now logged in the first time the user will be created in openshift AND a identity will be created.
For example User tom001 logging into openshfit.
oc login https://YOUR-LOGIN-URL --username=tom001
You will see the user with get user command
oc get user tom001 NAME UID FULL NAME IDENTITIES tom001 ID tom001 LDAP:tom001
and you get something similar from the identidy
oc get identity CW-OAuth:firstname.lastname@example.org NAME IDP NAME IDP USER NAME USER NAME USER UID LDAP:Tom001 LDAP tom001 tom001 ID
Now you delete the user
oc delete user tom001
So the user is not found
oc get user tom001 Error from server: users "tom001" not found
but in the identity the user still exists and can’t login.
You will need to delete the user also in the identity.
oc delete identity tom001
Now the user will be again able to login because in the LDAP the user isn’t touched.