You can build an app that checks orders for notes then tags, order tagging through API was released last month
For an integration in the admin submit a feature request https://ecommerce.shopify.com/c/shopify-discussion/t/feature-requests-here-52781
Order notes automatically get a nice little icon next to the order number in the shopify admin. I've attached a screenshot and you can see order #1322, #1310A have order notes.
Our Order Tagger app can read Order Notes and will analyse it to try and match a string that you specify. If found, it will add a Tag of your choice to the Order.