Yes, I recall various solutions to this problem being suggested before - I just haven't actually gotten around to doing anything about it, trying to decide which solution is more sustainable.
For adding a "classic" add-on category to the website, I don't know how well this will work as the collection of "classic" tracks inevitably gets larger. With this single category, let's say eventually we get enough really really good looking tracks in the main package that something like Amazonian Journey or Scotland gets pushed to add-ons. With a single classic designation, those would end up lumped in with "Classic Tollway" when they clearly come from totally different time periods and quality levels, so it may one day be desirable to differentiate between the two. [edit]I totally forgot about Gown's Bow, it's a much better comparison than Classic Tollway for this point.[/edit]
The better solution, I think, would be to be able to mark the version where the track started being included, and the last version it was included in. In the future, some categories could be calculated based on this too. I think there's already some math in-game to do arithmetic on version numbers from when the news system was implemented, so it shouldn't be too bad on the game side. I don't know a whole lot about the in-game code though. This info could be fairly easily added to the XML sent to the game from stkaddons.net, without breaking any compatibility with 0.7.x, but it wouldn't change the behavior of 0.7.x either. I could definitely get this done before 0.8 comes out, at least on the website side of things.
In addition to this (and this is something I've been thinking about doing, but avoiding specifically until I can sort out the huge mess in the uploader code properly,) the kart/track category could be changed from "Add-Ons" to "Classic" in the selection menus but I think that poses a few problems of its own - you may have noticed the current behavior in the game is that if you select a kart from a specific category, it only takes your opponents from that category as well. I'm not sure if this is possible, but it would then be ideal to put a kart/track in
multiple categories. I'm not really convinced this is worth the added complexity to the kart/track selection code in-game though.
Anyways, I think I'm done rambling for now. Because this will need to be supported in-game, I still need to discuss all this with the game devs (hiker, Auria) to see what they think is best/easiest on their side of things.
Thanks for your suggestions. If you've got any other ideas relating to this which I haven't noted above, I'm open to other ideas too. I do read the forum daily