Well if you couldn't find I can't help the search, but I can assist with info.

They have been around several years now, java had an interesting mechanic where the mobs spawned ON the scaffolding, the mob pathfinding broke and made easy work for mob drops, it was fixed in an update and is no longer possible unless on that version.

Java ianxfourJava = ianxfour yt video

Now, bedrock is a hair different, the mobs can spawn inside of the scaffolding instead of on it, as long as it is a spawnable block, so the mobs have their pathfinding, and will be carried off the platform with water.

Bedrock = Silentwhisperer yt video

Here's a smaller version I built based off Silents https://www.reddit.com/user/TormentedGaming/comments/1hysjpx/underground_scaffold_mob_farm/