Posts in a game's Steam news feed that Steam tags as patch notes.

News checked every 30 days

• An announcement is not a build. Developers can ship updates without a post, or post about several updates at once.
• Each check keeps the 10 most recent posts. Quiet games are checked as rarely as monthly, so a very busy feed can have gaps, and posts from before our first check may be missing.

Source: Steam Web API, ISteamNews/GetNewsForApp. Full methodology