I currently don't have a working mobile device but need to switch to 2FA for github.
The only way I currently know to do this is to purchase a Skype phone number and use it for SMS authentication. But that costs money and there's a risk that I'll accidentally let the subscription lapse.
Are there any no-cost ways of authenticating that don't require a mobile device or a working phone number?
if you manage to install android emulator or virtual machine on your PC (unfortunately, i cannot help with it) and than install https://play.google.com/store/apps/details?id=com.google.android.apps.authenticator2 on it, its possible to use this instance of Google Authenticator for tricking github you have 2 factor authentication. But i think this approach is something that degrades security dramatically. If your PC is stolen or hacked, they can access you authenticator and misuse it.
UPD: you can emulate 2factor authentication device with this library in nodejs (probably, analogous libraries exists in other programming languages)
https://www.npmjs.com/package/notp
this method can work - https://www.npmjs.com/package/notp#totpgenkey-opt
UPD: there is console application that does it exactly https://github.com/grijul/zauth
