For some reason the user_watching_first_post template isn’t listed in the list of templates that can be edited on the Admin / Customize / Email page, but the user_watching_first_post email type is still being used by Discourse:
The template can be edited from the Admin / Customize / Text page, but this is an overly confusing thing to have to do. In general, I find the way that email templates can be edited on both the text customization page and the Email page to be confusing:
For your case, it also seems that the changes that have been made are not backwards compatible. I don’t know if that’s something that can be fixed.
Again there’s the issue with backwards compatibility when upgrading from 2.8 to 3.11. Other than that, you should be able to edit the User Posted template from the Admin / Customize / Email page to make the changes you are looking for:
