I can not answer about the triangle ceiling pieces … all my items that disappeared were on sandstone triangle foundations.
Also can’t answer to what will happen to thralls on the triangle foundations.
A placeable is any item that you can place both those that you can pick up again (eg tapestry, bed, protected torch) and those you can not move and have to destroy if you want to clear the space (eg cupboard, large chest, furnace, carpenters bench)
And if the placeable is despawned when the patch goes live then anything IN those will also go bye-bye (can not confirm if you will get a loot bag drop that you could loot IF you were close to the despawned item or not … the large chests that went bye-bye for me were empty. Another on my server said he lost the contents when it despawned, but as we have a short time to grab loot bags perhaps he was too late to find it after he logged in)
So Funcom is trying to warn the console users to move anything they care about into storage on square foundations as there is a risk that chests, cupboards, vaults, workstations might disappear if they are on a triangle foundations. More warning than the PC community got, I am forever grateful to that tester who reported on the forum that his items despawned when he applied the test live patch to his test game as I heeded the warning and moved everything and this action saved half my resources and my first precious named smelter …ie half my storage chests and a furnace went bye-bye with the patch but were empty …alas I can not remember his name…