I have ran to the situation where I have to remove Facebook/Google authentication option from my app. So I want to know all the pain during this process. What about user data ? Is there any possibility to link newly created user with data from previous facebook-login-based user ? Any other situations which I have to fix ?
Oh my God. I personally do not think that you should do that. Yes, if their users in your application that have logged in with one of these providers, then yes their data possibly could be deleted. I mean if you have stored the access tokens to retrieve information about them, then you will be fine I think. But, if you remove OAuth2.0 then your users will not be able to log in with anything else, such as local authentication, due to the fact that they have not created an account via your local authentication system, as they do not have provided a password for their account. They only gave consent to read or write data about themselves, and you only know their email or username. Last but not least, a move like will definitely harm your User Experience (UX) throughout your application and your marketing as well.
I hope that helps! Try to search about some alternatives to solve your issues.