Conditional fields on content type with paragraphs I saw this question repeat in a few places without an adequate answer.

I have a D8 site that has a certain contain type ('deals') which contains an array of 'deal' paragraphs. 
Each 'deal' contains a 'deal_type' field and then a few content fields ('image', 'text', 'banner', etc.).

I would like to conditionally show the paragraph fields, according to what the user selected in the 'deal_type' dropdown.

I saw some answers mention a MODULE_form_alter() hook but this didn't seem to be called when I load my 'edit content type' page.

Can anyone help with this?
Yes you will have to hook form alter.
Make sure that you have done the following to make sure it's working.
1- create a module say it's called deals
2- add the deals_form_alter(...add the default params here ) In the deals.module file.
3- clear the cache 
4- add your edit form is in an if condition then right the following logic. (You can get the form is by dumping the $form['#id'].
5- use the states functionality or Ajax callback to hide and show the fields you want.

https://www.drupal.org/docs/8/api/form-api/conditional-form-fields
And https://www.drupal.org/docs/8/api/javascript-api/ajax-forms