I know Reel King has been around for over 10 years, possibly 15 now? Cleopatra is definitely 10 years as I remember seeing it for the first time in 2009. Chain Reactorz in 2009 too (made by the same coders as Reactoonz), although I think the Bejewelled / Candy Crush-type games are regarded as “instant win” games rather than slots. These are all on Sky Vegas where I used to dabble on back in the late 00s.
What about Break Da Bank? That looks pretty oldskool along with some of the other Microgaming.