Interesting, it's gained about 10% usage in the last year (~10% to 18-22% globally). Not bad!
This is why percentage is a bad measure when it can represent two things (percentage or percentage-points).
I like log-odds as a scale to use to represent adoption, going from negative infinity with no adoption through zero (at 50%) to positive infinity at full adoption, but with a typical range of -2 (~1%) to +2 (~99%) when using base e. (Although you can use any base).
By that scale it went from -0.95 to -0.65.
It's also interesting to note the difference between weekend and weekday IPv6 traffic (~18% vs ~20%). Businesses are apparently far behind consumers in adopting it.