Fuzic
From 2013 to 2020, Fuzic aggregated viewer numbers for StarCraft II, Overwatch, Hearthstone, Heroes of the Storm, World of Warcraft and Brood War live streams and turned them into rankings, overviews and statistics.
Because I was no longer able to spend as much time on it as needed to keep up the quality of the data, Fuzic stopped collecting data in February 2020.
You can download a dump of the data here. It contains the full database, as a MySQL dump. The source code of the site and its crawlers can be found on GitHub. Feel free to contact me if you have any questions about it. If you use the data for something interesting, please do let me know!