This was initially brought Upstream through Issue 13080 but nobody provided any tip after several months.

This is related to xccdf_org.ssgproject.content_value_login_banner_text.

Due to folding and formatting, spaces have to be specified in the banner as [\s\n]+ and this is totally understandable.
However it's unclear how to specify newline: \n doesn't work in particular.

There is a Jinja macro (shared/macros/10-bash.jinja) which replaces the escaped newline into a newline which uses a sed line:

{{# Strips newline or newline escape sequence regex #}}
{{% macro bash_deregexify_banner_newline(banner_var_name, newline) -%}}
{{{ banner_var_name }}}=$(echo "${{{ banner_var_name }}}" | sed 's/(?:\[\\n\]+|(?:\\\\n)+)/{{{ newline }}}/g')
{{%- endmacro %}}

The above sed line doesn't work at all for multiple reasons, checking the GNU sed manual :

• ?: operator is not a valid operator
• the pipe used to or the pattern should be escaped (|)

IMHO the following sed may be used instead: s/\\n|[\\n]/\n/g
This would match using \n or [\n].

Could you please explain what is expected from the user here?