There’s no reason you couldn’t keep track of that information in encrypted form and sync it across devices. Apple already syncs bookmarks, email, browser tabs (albeit broken of late), photos, contacts, calendars, and probably many more things I haven’t thought of, across all of my devices.

I don't get this view. Controlling information dissemination is like, half of negotiation strategy. If the counterparty knows everything you know, you're basically always at their mercy. Seems like a lot of trust to give literally anyone and everyone.