Is there any reason why Steam isn’t one of the built-in authentication providers?
There are more users for Steam (35 million according to SteamSpy) than GitHub (14 million according to Wikipedia page).
There is already a functional plugin.
This brings in gaming communities and customers to the paid hosting.
The plugin requires manual command-line intervention to get the API key from Valve saved to a config file, I presume that could be done through the web admin once integrated more fully.
I have not examined the code to see if this is trivial to take the preexiting plugin and integrate. Based on reading the tutorial for plugin creation I would guess that it would take a significant amount of rewriting.
Reading the plugin page indicates that the users email isn’t passed from the Valve API. Users would still have to enter that in manually and might not understand the benefit to them after having registered. Maybe these users will have already committed to the decision to register and choose another option or enter it in manually.
The Steam API for login could/will certainly change at some point in the futures and the Discourse interface will have to change to support it or remove support for it.
Despite the long list of cons here I feel like the benefits out weight them. But that could be because it will work better for my discourse communities to have it