Is or has smoking been permitted in the conclave ?
Possibly in the past. I doubt it now. The conclave takes place in the Sistine chapel and I don’t imagine that smoking is permitted there with all of that priceless artwork. I assume that even cardinals will take toilet/smoking breaks and go outside to have a puff.
I would be shocked also if none of the cardinals smoked. Too many Europeans in that group.
Not an answer, but I can’t help but remember that priest in Godfather III. Chain smoking holy man. 
Per this article:
Given that the Sistine Chapel recently underwent cleaning to remove the effect of 500 years of smoke (albeit from candles, not cigarettes), I’d be shocked if anyone would be so gauche as to light up in the chapel itself. But in their private quarters, or during the outdoor strolls which they’re now allowed in the Vatican Gardens, yeah, probably.
I think there’s a bit of confusion here between the Sistine Chapel and the Conclave. The area set aside in which the Cardinals and their “Conclavists” (the assistants they’re permitted to take in with them) are sequestered is a rather large part of the Vatican Palace. They meet for Mass in the Sistine Chapel, and may or may not be doing their deliberations there – a Catholic with more familiarity on the process can indicate where they meet – but they have individual suites and areas for private meditation, discussion, etc.; they’re not all locked inside the Sistine Chapel proper.