I still use this plugin myself, its my primary interface to the ML. I find and enqueue every song I play from it, and send songs to my ipod from it. Which is pretty much all I need it for.
So I wouldn't say its abandoned, it just meets my immediate needs so I havent bothered to put the effort in to update it.
It has a fairly complex code base, and is written mostly in straight C but really should be re-written in C++. I am reluctant to commit much more effort to it in its current form because:
a. The ML is undergoing a change to unicode in Winamp, yet ML_tree is all multibyte based.
b. I would be increasing the complexity of the code, increasing the need for a re-design/re-write, and at the same time increasing the amount of code that needs to be re-written/re-designed. Its a bit of a stalemate condition.
As I use it myself, it should at least remain usable. I do like this plugin, so its something I will keep on my todo list. If and when I ever get a large amount of free time to dedicate to this, I will probably re-write it (mind you I say that about everything).
I would like to see what the ML does with the unicode transition too. Already there are a few changes, such as with adding 'composer' etc, and a a few bugs fixed. With that, my existing ml_tree code base, and my growing experience with decent code design it should be a little more desirable to design / re-code / maintain.
The biggest problem is time, or lack there-of.