How can I create new custom region and publish content in a Radix sub-theme (Radix 6.0.0-rc4)?

I placed this under the "Support request" but this doesn't seem to be Radix specific issue, you can render a new region like any other themes the way that you described. We might need more info to be able to help you but I suspect it might be a template not being read correctly, eg. you might still referencing the Base theme rather than your Sub-theme.

That aside try reading the Radix documentation thoroughly, that might help.

Closing the ticket for now but feel free to comment

I think his problem is the new SDC. I currently struggle with that too. SDC makes it heavy complex.

In a subtheme you already have templates/page with two files page.html.twig and page--front.html.twog
Attention: If you delete page--front it will use the starterkits template file

You can edit the page or page--front and add your region there like this:

 {% block page_head_right %}
            {% if page.page_head_right %}
              {{ page.page_head_right }}
            {% endif %}
          {% endblock %}

But be careful that you override the core radix page like this. In my case i have copied over the contents of that file and added my regions, Example:

{#
/**
 * @file
 * Theme override to display a single page.
 *
 * The doctype, html, head and body tags are not in this template. Instead they
 * can be found in the html.html.twig template in this directory.
 *
 * Available variables:
 *
 * General utility variables:
 * - base_path: The base URL path of the Drupal installation. Will usually be
 *   "/" unless you have installed Drupal in a sub-directory.
 * - is_front: A flag indicating if the current page is the front page.
 * - logged_in: A flag indicating if the user is registered and signed in.
 * - is_admin: A flag indicating if the user has permission to access
 *   administration pages.
 *
 * Site identity:
 * - front_page: The URL of the front page. Use this instead of base_path when
 *   linking to the front page. This includes the language domain or prefix.
 *
 * Page content (in order of occurrence in the default page.html.twig):
 * - node: Fully loaded node, if there is an automatically-loaded node
 *   associated with the page and the node ID is the second argument in the
 *   page's path (e.g. node/12345 and node/12345/revisions, but not
 *   comment/reply/12345).
 *
 * Regions:
 * - page.header: Items for the header region.
 * - page.primary_menu: Items for the primary menu region.
 * - page.secondary_menu: Items for the secondary menu region.
 * - page.highlighted: Items for the highlighted content region.
 * - page.help: Dynamic help text, mostly for admin pages.
 * - page.content: The main content of the current page.
 * - page.sidebar_first: Items for the first sidebar.
 * - page.sidebar_second: Items for the second sidebar.
 * - page.footer: Items for the footer region.
 * - page.breadcrumb: Items for the breadcrumb region.
 *
 * @see template_preprocess_page()
 * @see html.html.twig
 */
#}
<h1>page html twig front my radix</h1>

{#{% include 'radix:page' %}#}

{#
/**
 * @file
 * Template for the main page.
 */
#}
{% set page_attributes = attributes ?: create_attribute() %}
{% set
  page_classes = [
  'page',
]|merge(page_utility_classes ?: [])
%}

<div {{ page_attributes.addClass(page_classes) }}>

  <div class="page-header">
    <div class="container">
      <div class="row">

        <div class="col-sm-6 col-md-6 col-lg-6 col">
          {% block page_head_left %}
            {% if page.page_head_left %}
              {{ page.page_head_left }}
            {% endif %}
          {% endblock %}
        </div>

        <div class="col-sm-6 col-md-6 col-lg-6 col">
          {% block page_head_right %}
            {% if page.page_head_right %}
              {{ page.page_head_right }}
            {% endif %}
          {% endblock %}
        </div>

      </div>
    </div>
  </div>

  {% block page_navigation %}
    {% include 'radix:page-navigation' %}
  {% endblock %}

  {% block page_content %}
    {% include 'radix:page-content' with {
      page_main_utility_classes: ['py-5'],
      page_header_container_utility_classes: ['mb-3'],
    }%}
  {% endblock %}

  {% block page_footer %}
    {% include 'radix:page-footer' %}
  {% endblock %}
</div>

Works in this case. What i have not achieved so far is adding stuff to the html sdc because just copying that over will not work.