I think it would be a nice addition to be able to write a note / description to a .litematica file. The reason I would like to have this added is so it would be possible to write some 'heads-ups' about the schematic or what the rates of a farm are, so I wouldn't need to keep a separate list, but can just check the notes in the litematica menu.

Right now if you look into the 'Load schematic' list and select a schematic you see info about who made it, when it was created, how many regions it is, the volume, block, size, but being able to write note there would be nice.

The SchematicMetadata class has actually already always had a description String field, as it was always intended for it to have a description text area. It is simply not yet possible to edit that value in-game, and it's also not shown anywhere yet. But it is on the todo list and will hopefully be implemented in the next big update, as I have meant to add it in many occasions but just never got around to it.