Wordpress - Where does wordpress store the FTP credentials?

It doesn't. WP Filesystem API will ask for FTP credentials, but it will do so for each operation. They aren't stored persistently.

They can be stored persistently by hardcoding into wp-config.php, but WP won't do that itself, it's a user action.

Note that WP only asks for FTP credentials if it cannot modify files without them. If specific setup used to ask for them and stopped then likely the server configuration or file permissions had changed and now allow WP to manipulate files without going through FTP.