Currently, Plasma supports custom metadata for objects. This doesn't seem to be used at the moment, and removing it will simplify the interface and implementation of plasma. Removing the custom metadata will also make eviction to other blob stores easier (most other stores don't support custom metadata).
My personal use case was to store arrow schemata in there, but they are now stored as part of the object itself.
If nobody else is using this, I'd suggest removing it. If people really want metadata, they could always store it as a separate object if desired.