“Is God in Purgatory?”
Never thought about that, but, as far as I know, no, God is not in Purgatory. That absence, however, is temporary and the soul is consoled knowing it will soon be united with God for all eternity.
“Whatever happened to Limbo?”
Nothing. You can still believe in it if you want. It was never a defined dogma in any case.
“If Limbo was truth at one time, when did it stop being truth?”
No one has said it is not true. Thus, your question is based on a false premise. Maybe you should study up on the issue.
Ahhhh another infallible teaching from the magisterium .. into the pit ..