The source data seems to be here:

https://github.com/web-platform-dx/feature-set

There’s a bunch of YAML files in the ‘feature-group-definitions’ directory and the ‘leaf node’ ones have ‘is_baseline’ keys in a ‘status’ dictionary. See e.g. https://github.com/web-platform-dx/feature-set/blob/main/fea...

I am very much struggling to find anywhere that actually lists versions currently counted as ‘baseline’, or any definition of what a ‘major version’ is though.