The UDP announce protocol has VERY little support outside of the original development. (Totalling 3 client packages and 3 tracker packages at last count, one of each seems to have fallen into an abandonware state).
Several functions of the UDP announce protocol are rebuilding what is already present in the HTTP(TCP) announce STANDARD, including security and data arrival reliability.
Bandwidth requirements for pure trackers right now are low enough that people on residential connections are running 10k+ peer trackers without affecting their browsing.
Weeding out the bad clients and preventing them from even connecting is the better option to reducing load.
Older versions of BitComet and uTorrent were found to be abusive against multitracker networks and trackers with multiple listen ports (improper multitracker implementation. I reported it to both and it was corrected.)
Requiring clients support compact announce is the biggest way to save bandwidth over older announce methods. (
reference comparison)