How to show forums even if you don't have access?
Implementing custom permissions in Drupal 8:

https://www.aram.cz/article/custom-permissions-drupal-8

You should be able to use static permissions, which are configured in a YAML file in your custom module. From the article:


  To declare static permisisons, you need to create a file in the root directory of your custom module and name it YOUR_MODULE.permissions.yml.


'sample permission 1':
  title: 'First level permission'
  description: 'Gives first level permission to the users.'

'sample permission 2':
  title: 'Second level permission'
  description: 'Gives second level permission to the users.'

'sample permission 3':
  title: 'Third level permission'
  description: 'Gives third level permission to the users.'
  restrict access: true


Then you can check for that permission in PHP code like so:

if (\Drupal::currentUser()-&gt;hasPermission('sample permission 1')) {
  // The "sample permission 1" has been granted to the current user.
}
else {
  // The "sample permission 1" has NOT been granted to the current user.
}


I'm not sure exactly how you'll use this technique in your situation to allow users to see only a summary of forum post titles without the ability to click through and see the individual forum posts, but hopefully it will set you on the right track.