This update, beta 184.108.40.206, is the seventh beta version for a future patch to 1.16, but also contains features for the upcoming Caves & Cliffs update, hidden behind a toggle.
As described in an article on Minecraft.net, this beta adds a variety of copper blocks to the game.
Copper is obtained from copper ore, which can be smelted into copper ingots. These ingots can then be used to craft copper blocks, which can then be polished or cut. All of these crafted varieties can be turned into their respective slab and stair variants.
As well as these decorative blocks, another block that can be crafted from copper ingots is the lightning rod, used to control where lightning will hit, useful for diverting it away from flammable blocks.
Copper ingots may also be obtained from killing drowned zombies, which previously dropped iron ingots instead.
Copper originally game to the Java Edition late last year in the first snapshot for the Caves & Cliffs update.
As usual, this beta comes with more than a few bug fixes, including fixes to the screen reader and structure block usage.