theres a load of different ways you can do this depending on on different aspects of your game,

for instance the first one i can think of is, and for me the quickest to do is assigning an invisible trigger roughly the same size as your platform if anything just a little bit over.

then set the hight of this trigger to whatever the maximum height the object can go to onto this platform, and of course parent it in case ur platform moves or just to remember its a part of that object. then put in a simple on trigger exit line destroy.

then of course with this if you wanted the platform to fall after a couple seconds u could use on enter yield then play animation, then destroy.

just a quick idea