I don't disagree with ThudnBlundr but we went into the battery technology in great detail in the early years of this forum (not sure if threads are still available) so I am relying my amateur understanding and what aged memory can recall.
As I remember, whilst it may be lithium batteries do degrade over time anyway, it was shown that the ultimate limiting factor is charging cycles, as I described. No doubt individual usage, including too frequent rapid charging, may have some effect on this, but it would appear to be only marginal.
In the last 7 years there have only be a handful of suspected battery failures and only a couple of warranty replacements, in the early days, that I remember - i.e. manufacturing faults, which would have already surfaced in a second hand car and dealt with.
No, I haven't seen any "proof" about the BMU using the buffer with age but this was suggested as a reason why Mitsu have the buffer at all, rather than offer a longer range when new. One of our departed Aussie contributors did experiments with trying to run the batteries flat by deliberately running out of petrol. As I recall even though showing "No Charge" the car went back into EV mode once the ICE died but he still didn't get to exhaust the batteries - he only got a few miles in Turtle mode down to about 10% capacity.
So unless there is some technical reason that has not been explained here why there is a buffer other than as a reserve to maintain a useable range, why is it there? Of course, very few drivers who measure capacity would have needed it yet (given their relative newness) - so most of us may be using the buffer but don't realise.