Degradation or change in software?

[lymex2018]

Not all vehicles have a below zero mile buffer. And it certainly isn’t the same on all vehicles. Another S60 could very well only show 1 kWh remaining at 0 miles. Another could show 0 kWh at 0 miles.

It might be small like 1kWh, but so far I've seen no evidence. All the buffers below zero I saw are larger than 2.5kWh.
I'm quite sure that there used to be no below zero buffer, but owing to the zero estimation error, there exist cases where some cars/some times run out of juice when the dash still display none zero. Then Tesla modified the display more conservatively so that there is less chance it will happen. That is why the internal state of charge is still named SOC UI, but actual SoC displayed on the dash is smaller.