I like the look as well. I understand the caution expressed above, and would never place fresh leaves, on or off the branch, into a tank.
I think you would be safe if you waited till they are dead, but before they actually drop off; a pretty brief window of opportunity, to be sure. I always pick mine up shortly after they drop from the tree on their own.
Wait...what? Are you saying that same branch with leaves on it has been submersed in your tank for over 2 years???
Those look like maple leaves, one of the types I use regularly. Dead/dry maple leaves added to my tanks take a day or so to sink...and then deteriorate to the point of invisibility with a few weeks. I can't imagine how on earth you get years out of those!