If you're thinking 'wasn't Runescape already on mobile', you might be thinking of Old-School Runescape. OSRS, as it's commonly referred to, is a spin-off of Runescape based on its classic 2007 build - it was launched in 2013, and in 2017 a mobile version of that was launched....
"I'd say one of the several experiences when exploring that area [in terms of development, not as a player] was 'what does Runescape mean' and 'how much of that essence did we want' - did we want it to be a very linear experience? How much choice and deviation did we want? ...