After Infinite Crisis he was away for one year in-universe. Not in publication time. The 52 event that was coming out filling the gap in-story. But concurrently the Batman titles remain coming out. I don’t know if that clause expired, but Bob Kane had one that if Batman ever went out of print the rights would revert to him.
@Leostales Yes, the time he was hit by Darkseid's Omega beams. It's possible that they can kill Joker for a few years before bringing him back, if they don't want to make it permanent.
Absolutely not. He is one of DC’s most valuable assets, second only to Batman and Superman. Even if they do kill him off, he’ll be back in under a week.