It would be great if you could put a little tick next to any routes you’ve already completed
I love to see my progress and build up my XP, and routes are a great way of doing it, along with lots of miles! But knowing which routes I’ve done when I’m about to ride would help me choose a new route to take, but I have no way of knowing.
Perhaps a tick next to the route name when you are about to choose one would be enough.
As Zwift has ignored this request for years your best bet is using a 3rd party site. I use ZwiftHub.com to keep track of which routes have been completed. Also believe zwifthacks.com has a feature like this too.
You can also download a checklist from Zwiftinsider to check off the routes. printable-list-of-zwift-routes-by-difficulty
It makes it easy to see which ones you are missing.
Then you don’t have to login on Zwifthub. (I use Zwifthub too).
Zwiftinsider is a great site to visit with a lot of info about Zwift.