TF2 Kart - what happened to it?

If the TF2 Kart you're thinking of is the one I know, it's only two years old, from Halloween 2014, when they added bumper carts. It was pretty stable.

I think everyone got turned off by it when Valve introduced bumper cars to TF2 and they turned out to be utterly atrocious in every single way.

Does this guide describe what you're thinking of? It has a list of servers running this map at the bottom but I didn't check if they are still up.

It should be noted (as far as I'm aware) TF2 kart maps were linked to the bug where you'd fire twice when clicking the mouse (only client-side though, only one gets sent through, but VERY annoying).

There was a kart map played on a game day a couple weeks back, it was fun, but only for like 5 minutes. To me, it's kinda just a novelty, and not something I would put hours of work into.

It turns out a first-person shooter with no way to significantly modify the source code is a poor basis for creating a game in a completely different genre. Imagine!

The Halloween kart controls like ass

I was able to get a decent fun kart map working, but it ran into a few issues:

A) To my knowledge, you can't control what direction people will spawn into their carts at. After the first round on the map, everyone will spawn into their karts facing whatever direction they were when the previous round ended. This might be fixable, but it was annoying.
B) I couldn't find a reliable method to remove the parachute from the jump spell. This might also be fixable? I was able to accomplish it with a point_clientcommand, but those don't work on all servers.
C) Double-shot bug as mentioned before. Testing a kart map basically means sabotaging an entire map test.
D) It gets old after a while. It doesn't lose its novelty as fast as medieval mode does (for me!), but I don't think it's a sustainable mode. Interaction between players is fairly limited, especially when you're limited to two teams - the best I could come up with was a time-trial mode where the first team to X laps would win.