This is a simple mod that allows vanilla's reinforced deepslate block to be placed horizontally (as that is not possible in vanilla Minecraft), which results in more possibilities when building with the block. The block can be placed in all four horizontal directions. The direction is determined by which block face the block is placed on.
This mod does not alter ancient cities. The frame in the middle is still made of upwards-facing reinforced deepslate, and that will not change.
This mod does not add a recipe for reinforced deepslate, or make it otherwise available outside creative mode, and it's not its job to do so.
You are allowed to use this mod in any modpack.