How can I mark itineraries that I don't want to delete?

You can always use our Itinerary Status field and add a note there.
 
You'll be able to see it as you scroll through the itineraries in your feed.
 
Here is a recording (no sound): https://recordit.co/5xUr80gUuL

Click the GEAR icon to edit the itinerary.
Scroll down to "Itinerary Status" above the travel dates.
Write your note in that box.
Click the "SAVE ITINERARY" button.
You'll be able to see your itinerary status in the title box.