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Recently reported some issues involving /etc/login.defs util-linux, pam
and shadow-utils. See bz#1884702 and bz#1884695

While doing an unrelated test on the test environment of the above issue,
I just noticed the @summary message when adding a test user with 'useradd',
and using LOGIN_PLAIN_PROMPT in /etc/login.defs.

Feel free to reassign to shadow-utils if that is the case.

This is necessary to fix in shadow-util. It seems RHEL-8 has already a patch for this issue.

Note that we're trying to fix it also in upstream level and there is a shadow-utils pull-request to add all missing options. 
For more details see: https://github.com/shadow-maint/shadow/issues/276

Is this an actual error message or just a warning? I mean, where you able to add the user?

It is just a warning message. User can be added normally.

Since this is a cosmetic issue for RHEL7 and it has already been fixed in RHEL8 I'm closing it as won't fix.