> penalise you for downgrading by deleting random bookmarks and your browsing history

aka they don't support profile data migration between versions. "Penalise" is needlessly polarising.

On the one hand yes, but on the other hand prior to 55 it worked fine.

I feel like they could have saved that up for 57 as part of the cake icing.

Actually, they never supported backwards migrations: it just happened to work relatively well. They even explicitly noted this in the release notes. And it's easy to imagine that it would cost a lot of effort, especially with the massive amount of changes in 57, to support something that would be used only once by a relatively small number of users.