Hi!
Currently, cave-ins use an extremely simplified mechanism that as long as there is one connection via horizontal or vertical neighboring floor/wall tiles to the map border or an undisrupted connection of a similar nature to the bottom of the map, a cave-in will not occur. Thus, as long as you have even a single tile connection, you can dig out rooms as large as you like. Even hollowing out entire layers of the map are no problem.
So, in the current versions (40d, 31) you do not need supports for your excavation. In addition, as far as supporting is concerned, a constructed wall does the same.
However (unless this has changed in recent days as I have rarely used it), there is one difference: you can use a mechanism to build a lever anywhere inside your fortress and then link that support to the lever using two more mechanisms. If the lever is pulled after the link is complete, the support will collapse. This way, you can artificially cause a cave-in without risking the lives of your dwarves:
Dig out all tiles below the floor you wish to drop
Build one support somewhere below the floor you wish to drop connecting it to the lower floor.
Link the support to a lever.
Dig channels around the area you want to drop so that it is only held up by the support.
Get your dwarves away from the area.
Pull the lever.
Deathworks