The Terms List Core Block can be used to display a list of existing Forums on the frontend.
While editing a page with the Block Editor:
- Add the Terms List block
- Select Forums under the Taxonomy option field.
Please note that due to a WordPress bug, the option to Show empty terms should be enabled for the terms to display on the frontend. Alternatively, the shortcode [groups_forums] can be used, that gives the same result on the frontend.