This site uses data from https://github.com/mevdschee/aoc-stats. Maurits created a tool that collects data from the advent of code and visualizes that. I have grabbed the file medals.json from his repo and used this data to calculate the puzzle complexity.
The data in the file shows the time it took the top 100 participants on the global leaderboard to collect the two medals for every day, if they made it that day. I calculate the complexity as the sum of the average score for the first medal and the average score for the second medal.
We can of course debate whether this is the best way to assess the complexity, but hey, this is just for fun. And if my method really annoys you, feel free to submit a PR with an alternative algorithm. (See above for the link to the repo).