How do I find out what the %{respond_instructions} replacement tag contains?
Can it be moved to the top of the email? Where is there a reference for the contents of all these tags?
You can move the respond instrictions to the top by editing the email templates. You can do that at /admin/email/templates. I think you can also do it at /admin/customize/site_texts which might be a little easier as you can use search to get a list of templates containing the term.
I used AI to generate a list of the keys that are used to populate respond_instructions based on discourse/lib/email/message_builder.rb at main · discourse/discourse · GitHub and this is the list I got. I did not verify the result, but thought it might still help you.
user_notifications.pm_participants
user_notifications.only_reply_by_email
user_notifications.only_reply_by_email_pm
user_notifications.only_reply_by_email_pm_button_only
user_notifications.visit_link_to_respond
user_notifications.visit_link_to_respond_pm
user_notifications.visit_link_to_respond_pm_button_only
user_notifications.reply_by_email
user_notifications.reply_by_email_pm
user_notifications.reply_by_email_pm_button_only