Petrified Oak Slabs, also known as "Alpha Slabs", are a legacy item, still present in game, but unused. You can access it from the Creative menu by default. But it's unavailable in survival...

I changed that last part. With this datapack, I added a way to have those slabs. Just put regular oak slabs in a blast furnace (and only a blast furnace), and "smelt" them to convert them to your brand new petrified oak slabs. I hope it's balanced yet simple enough, otherwise let me know in the comments!

Why would you want that, you might ask? You see, oak slabs in Alpha behaved exactly like stone slabs. It is still true with the "petrified" ones you can access in your creative inventory. So, you can have the benefits of a stone slab in your world, with the cosmetics of an oak slab and without the risk of burning them! Be careful though: as a "stone" slab, you should mine it with a pickaxe and not an axe.

Just put the datapack in your world's datapack folder. CAREFUL, though: as much as I doubt Mojang would remove such a small legacy item, you should keep in mind that they are still allowed to do so, and could potentially corrupt your world with a new update! You should (really) backup your world before updating them anyway to prevent any issues though!