Hey xenollo,
Yes, you should be able to see chests at bedrock level with ChestESP, assuming normal world height and render distance settings.
By default, Minecraft servers have their render distance set to 10 chunks and their world height set to 384 blocks. This is enough for ChestESP to detect chests at bedrock level even while you're at the top of the world.
If your server has a much taller world or a much shorter render distance, then you might no longer be able to see chests all the way down to bedrock.
Hope this helps!
