It should be noted before a bunch of folks translates the comments about blocked bottom entrances as some justification to run out and put in top entrances, that this is the reason that the old style wood entrance reducers always came with the suggestion of making sure the slot was "up" for the winter.
With a little thought being used, issues can be overcome as they have been seen many times prior by other beekeepers. It is amazing to see basic wisdom such as rotating the slots on the wooden entrance reducer to the up position (which they can be left all year long) simply forgotten by so many.
Many of the newer metal reducers with staggered holes take into account dead bees and it would take a massive deadout to clog the entire bottom entrance area. If that were to happen, you got bigger problems than a clogged entrance.
BTW.....I have never seen a hive have problems with dead bees completely clog a entrance to the point that it could not be overcome by the bees, except with massive deadout by starvation or disease.
I have however have seen dead bees actually be piled up at the door, with the placement of so few bees, it made you realize it seemed that the bees clogged the holes on purpose maybe in attempts to block off air flow or for some other reason. I also have some boxes with screened holes, and for some hives that do not propolize the holes shut, they will sometimes plug them full with dead bees blocking off any air flow.