The way it is drawn: separated liquid refrigerant and any residual oil never gets into the large riser's trap unless you have a real serious overflow, due to the FOT reducer ...So on the restart of something like a spiral or similar process freezer: residual oil from upstream return line will be in the small trap. Assuming for a moment that traps and risers are all in a surrounding at higher temperature than the evaporator temperature: oil has means to return during pull down or otherwise high load because there will be a pressure drop through even the big riser.
In an overfeed arrangement: if you take the small riser off the top of the main, the upstream pipe will start to act as separator and you will get some unmanageable liquid levels at the pump vessel...Depending on sizes, lengths and similar.

We don't have any real use for the collection of fittings illustrated...Or put more succinctly: We don't do it this way.