I'm not an expert in the area and don't know the nuances as much as I'd like, like I've wondered if there's a way to have 2 keys - so in the case of blackmail, you give a different key than normal, which will work - however then there's a way to trace.
Obviously the best system is having a system of governance that is stable and accountable. The situation to perhaps design for is say tyrannical dictatorships that may rise.
I suppose the most important aspect of git would be redundancy of data, along with perhaps immutable and/or offline archives; evidence may be changed without anyone ever knowing though to check, unless perhaps there were regular (every 5 years?) consistency checks between archives and present-day/online historical data?