> If you're working online (eg figma), you can just download history lazily.

You can download the history lazily, that's a special case of incrementally loading the document.

If the history is only history, then sure. But I was understanding that we were talking about something that may need to be referenced but can eventually be GCed (e.g. tombstones-style). Then lazy loading makes user operations that need to reference that data go from synchronous operations to asynchronous operations. This is a massive jump in complexity. Incrementally loading data is not the hard part. The hard part is how product features need to be built on the assumption that the data is incrementally loaded.

Something that not all collaborative apps will care about, but Figma certainly did, is the concept that the client may go offline with unsynced changes for an indefinite amount of time. So the tombstones may need to be referenced by new-looking old changes, which increases the likelihood of hitting "almost-never" edge cases by quite a bit.