I just put a suggestion in factorio forums about linked belt which would permit UPS-friendly transfer between surfaces with belts. I think this is a kind of thing some mods like factorisimo would really benefit.
https://forums.factorio.com/viewtopic.php?f=6&t=93017

I've tested the functionality, linked-belt are now a thing in factorio 1.1.6

place two linked-belt (they have a speed equivalent to a basic-belt)
set input_belt and output_belt on the two linked belts
do output_belt.linked_belt_type="output" on output belt
do input_belt.connect_linked_belts(output_belt)

items can now pass through linked-belt

I'm aware of linked belts. I'll definitely be using them, but it'll take a week or two until I have time to implement it along with some refactoring.

Was it implemented?

It's not implemented yet.