Re: synced github history
This refers to the subset of PRs that will be displayed in your PR inbox, as well as the time period for which you'll be able to see insights for.
ex. if you're on the free tier, you'll have 3 months of synced GitHub history, so you'll be able to view PRs in your inbox that have been updated within the last 3 months. You'll also be able to see insights for the last 3 months.
more on pricing here:
https://graphite.dev/docs/pricing-and-plans