a WalledGarden concept that I don't see as existing (any more?) in my version. Is there a means to achieve this result in Elgg 2.3.7?

Restrict pages to logged-in users == Walled Garden

More on http://learn.elgg.org/en/stable/guides/walled-garden.html

I thank you very much for your prompt answer.

Does this mean that I need to develop my own plugin hook to make elements of information publicly visible? I would assume that the variable to be changed by the hook is the visibility of the information. Is this right? Does this variable have a name? What are the values it can have?

Does this mean that I need to develop my own plugin hook to make elements of information publicly visible?

That's right. You can start with this skeleton.

Does this variable have a name? What are the values it can have?

Just follow instructions and make something like this:

elgg_register_plugin_hook_handler('public_pages', 'walled_garden', 'my_plugin_walled_garden_public_pages');

function my_plugin_walled_garden_public_pages($hook, $type, $pages) {
   $pages[] = 'blog/group'; //For Blog plugin
   $pages[] = 'pages/group'; //For Pages plugin
   $pages[] = 'event_calendar/group'; //For Event calendar plugin
   
   return $pages;
}

Check out the full code with using the mentioned skeleton plugin.

But I think that it willn't work because Walled Garden removes ACCESS_PUBLIC and set ACCESS_LOGGED_IN default access level instead.

So 'blog/all' will be work but not 'blog/group' in this case.