I know one quickest solution for that. 

• Activate developer mode

• Go to Settings => Technical => Subtypes

• Find subtypes for model project.task

• Open subtypes one by one and set "Internal Only(internal)" field True
  

• Now it will create only system notification but it won't send any email to any follower. 
  

• 
  

Make sure before doing this, it will not send any emails to any followers for those internal types, so be careful if you are using odoo emailing system to notify project stakeholders.