This is the description from the menu:

A famous cocktail since the early 1900s, made from dry gin, DOM Benedictine, Cointreau, cherry brandy, Angostura bitters, Grenadine, lime and pineapple juice

It seems as though it's now part of the economy class menu, as both the menus on my recent flights mentioned it. It hadn't been a feature on my flights last year. But given jef7's and my experience, it might run out on the segments heading back to Singapore. The FA used the words, "I'm sorry, sir, but we are unable to offer the Singapore Sling on this flight." (Followed of course by, "I apologize for not telling you in advance. Would you like another cocktail?" Ah, the SQ service that I love.)