This page is just a reference of all the known Steam APIs, I do not know how they work. Please do not email me with questions.
UNDOCUMENTEDPOSTDeregisterMobileDevice
https://api.steampowered.com/IMobileDeviceService/DeregisterMobileDevice/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
string No
UNDOCUMENTEDPOSTRegisterMobileDevice
https://api.steampowered.com/IMobileDeviceService/RegisterMobileDevice/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
string NoMobile device ID string sent up by the mobile device, e.g. 'GOOG:abcdefg...' or 'AAPL:123456...'
string NoLanguage the mobile app is configured for (see s_LanguageNames::m_pchShortName for acceptable values).
bool NoTrue if push is enabled on this device, false otherwise.
string NoSteam mobile application version string, e.g. 1.2.1
string NoMobile device OS version string, e.g. 6.1.0
string NoMobile device model, e.g. iPhone, iPad, Nexus 7
string NoIdentifier used by two-factor provider
int32 NoWhich app is this?