Around 6.3V was a standard voltage for vacuum tube heater pins (the part that emitted free electrons via thermal emission inside the tube to be available for the electrical fields to accelerate).
My guess would be that historically 6.3V supplies made forvacumee tubes where commonly available when transistor based electronics where created so it made sense to utilize them. Works quite nicely for the typical 5V circuits as a "rough" input voltage to be feed through a LDO regulator, for example. And so it just stuck.
"encouraging unrest in politically sensitive climates" - La Résistance in Vichy France would fall under this. In some countries asking for fair and democratic elections would fall under this.
Or one could say that oppression advances by one funeral at a time. It is not like there haven't been examples of societies changing towards more totalitarian and oppressive over time.
I find it shocking how common this line of tough is now. Maybe the world dreamed about by people who actively celebrate the deaths of those who don't agree with them, is not such an utopia as we like to believe.
My guess would be that historically 6.3V supplies made forvacumee tubes where commonly available when transistor based electronics where created so it made sense to utilize them. Works quite nicely for the typical 5V circuits as a "rough" input voltage to be feed through a LDO regulator, for example. And so it just stuck.