Stats, passive values and prices are read from each patch's own game files. Item names, recipes and tooltip text come from Riot's Data Dragon. Every patch is compared with the one before it.
Passive values keep Riot's own internal names, split into words, such as Base Damage or AP Ratio. Ratios are decimals, so 0.05 means 5%. Green and red only mark cost and stat changes, where more or less has one meaning. A longer cooldown and a bigger shield are both bigger numbers, so passive values stay uncolored.
A passive value marked no longer listed has left the item's data under that name. Usually Riot renamed it or rewrote the passive, so check the tooltip text before reading it as a cut. Changes that live inside a formula rather than a named value, such as Bastionbreaker's damage in 26.15, only show up in Riot's patch notes.
Hotfixes aren't tracked separately. Tooltip wording is shown only for an item whose numbers also changed that patch.
