1 extra level of security? Perhaps the added cost of the system doesn't warrant it, perhaps there are better ways to mitigate.
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?