Extra Pieces is a mod for the Fabric Mod Loader for 1.14+ versions of Minecraft.
It adds Stairs, Slabs, Walls, Fences, Fence Gates, and more for any block you want. This includes vertical slabs named Sidings and sideways stairs named Corners, and a few other things.
And yes, when I say 'any block you want', I mean it.
By default, the mod creates all of these 'pieces' for a considerably large amount of vanilla blocks, but it creates them based on json files I call 'piece packs'! (found in the config directory)
The default Piece Pack includes most vanilla blocks you'd want pieces for, and other mods can include piece packs of their own. (Terrestria, for example, comes with a piece pack!)
Have fun with unlimited block shapes!
An additional addon, which adds Buttons, Pressure Plates and Infested Blocks for each piece set, is available here.
DISCLAIMER: This is NOT a Forge mod! It does not work with Forge. It uses the Fabric Mod Loader (found here) in addition to the Fabric API. It will most likely never be ported to Forge. Do not ask for a Forge port! You will be ignored!