Answering on behalf of PlanetScale (I'm the CTO!). I don't remember exactly what was different from upstream, but it wasn't a whole lot of changes.
Fortunately, PlanetScale runs a well-maintained fork ourselves, so we're very used to taking custom changes and getting them deployed. In this case, we asked the Block team for all of their changes and went through them one by one to see what was needed to pull into our fork.
By the time we did the migration, we made sure that any behavior wouldn't be different where it mattered.
Mostly the diffs were related to running against the on-prem MySQL instances smoothly: stuff like changes to split tooling or how you boot up the pieces. We have had unique vindexes or query planning changes in the past but we either deprecated or upstreamed them prior to migration.
Fortunately, PlanetScale runs a well-maintained fork ourselves, so we're very used to taking custom changes and getting them deployed. In this case, we asked the Block team for all of their changes and went through them one by one to see what was needed to pull into our fork.
By the time we did the migration, we made sure that any behavior wouldn't be different where it mattered.