Interesting question!
And I too am curious to see answers.
I just bought a 2015 CPO TDI Golf and it has the RNS 510 nav system with the MDI port in the small compartment under the head unit.
Having gotten spoiled by my Alltrack Car Play, I didn't realize how dated the RNS 510 was until I used it. It's serviceable, but our phone nav is now SO much better and there are three decent options: Waze, Apple and Google. All with better graphics and all are continuously up-to-date so no updates required.
So - your question is intriguing. My car came with the old ipod - MDI cable, which works OK on my old Ipod. But certainly NOT my iPhone 8.
i've see the head replacement units - expensive and I really don't want to tear my car apart. I love the car but wish VW had gone ONE MORE YEAR with the TDI and then our CPO cars would have had the later head units with Apple Car Play!
So - Has anyone tried to plug their iphone or Android into the MDI port through the USB cable, and does it project the phone screen? I'll be shocked if it does, but if so it will be a simple and cheap way to get our phone nav!