How to remove the "high priority" flag from incoming mail in macOS mail
Hmm. This is theoretical and untested but according to this:


  Many mail clients have the option of marking an outgoing message 'high
  priority' or 'urgent.' In Mail.app, such messages show up in the Inbox
  with two exclamation marks (!!) in the flags column. If you want to
  sort messages by the flags column, so that messages you flag yourself
  are at the top, all high priority messages you have ever received will
  also be at the top of the inbox. 
  
  Mail.app currently has no way of removing this flag from emails you
  have received (as noted in this Apple discussions thread.). The
  following shell script allows you to unset the priority status of all
  the emails you have ever received, so that no emails in your inbox
  have '!!' next to them any more:

sqlite3 ~/Library/Mail/'Envelope Index' 'update messages set flags = flags | 196608 where flags &amp; 196608 != 196608;'

  
  You can save this as an AppleScript by opening AppleScript Editor and
  entering the following:

do shell script "sqlite3 ~/Library/Mail/'Envelope Index' 'update messages set flags = flags | 196608 where flags &amp; 196608 != 196608;'"



You get set this to run when mail receives an email marked with high priority:


Save your script to `~/Library/Application
Scripts/com.apple.mail` Mail > Preferences > Rules > Add Rule


If any of the following conditions are met
Priority is High
Preform the following actions:
Run AppleScript [Select Your Script]