Out of the mainstream formats, JSON is among the easiest to parse - thanks to the CLI tool jq. Does UCL have something equivalent?

Protobuf (or any other binary format) has the benefit of being faster to load, but I agree - it should not be be used to store things like history. It should be easy to parse history without having to write code. And with Protobuf, you need the schema.

If a binary format is going to be used, it should be optional - or perhaps there should be tools to convert the history file to/from a plaintext format.