Get taxonomy field value in forum-list.html.twig
Your question is very vague.
I suggest setting up xdebug (https://www.drupal.org/docs/develop/development-tools/xdebug-debugger) then using the function devel_breakpoint() to be able to see how to reach the values/fields of your entity.
If your taxonomy is attached to your topic, I'm assuming that the solution will be something like forum.taxonomy_field.value.
If your taxonomy is not attached to your topic, you should have a look to preprocess your topic to add the missing data by using template_preprocess_forum_list()   I guess?
Here is the list of all preprocess hook, maybe another one with forum might do the trick: https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Render%21theme.api.php/function/implementations/hook_preprocess_HOOK/8.2.x