They are hard to get but do happen only if there’s not a single elite still on the waitlist and there’s seats available. During the 1st half of the pandemic though, it was insanely easy to buy up at check in as most elites weren’t flying. Even after they stopped doing first at half capacity with every seat next to you blocked, I still experienced many flights with spare seats and people unwilling to pay the upgrade fee. Nowadays, it seems like we are back to pre pandemic trends so if you do get offered one, buy a lottery ticket too.