Previously, prismarine stairs, slabs, and walls were a part of the forge:supports_conduit block tag. This tag was eventually replaced in 6b06bf6 with the method isConduitFrame but that commit dropped support for stairs, slabs, and walls made of prismarine materials.

This was intentional.
And there's no need to put [1.16.1] in the title of an issue since it's currently the only publicly supported version of UA.
For beta issues, report them in the discord server.