I would like to add a couple paragraphs of predefined text (not an input field) to my contact page. What are my options to do this in Drupal 8? Until this point I've been using JS to get the job done.
You can create a module, hook on hook_form_alter and add a #prefix to the first input field.
More simple: Create a block with your paragraphs and switch it on for region "(main) content" on the contact page (no programming needed).