> If you remove these ops from history, does that remove the ability to time travel (per the home page "An antidote to regret, enabling historical edits traversal") or merge branches?

Yes. But squash can be supported.

> How can we be sure an operation is synchronized?

In Loro, we not only record the real-world timestamp efficiently, similar to Git, but also capture the DAG information. This approach ensures that if an operation (op) is particularly old, it will have many other ops depending on it. By utilizing both pieces of information, we can determine the operations that are likely synced across all peers. For peers like servers, it's feasible to preserve all operations. However, we can remove some operations in scenarios such as opening the document online for the first time.

> If dropping these ops is necessary for speed/storage optimization but disables time-travel, is it possible to put the removed historical/tombstone ops into a "cold storage" that's optional and only loaded for time-travel use?

Yes. This is not supported at the moment, but we hope to implement it before version 1.0.