Well I mean, they had franchise lore. they just keep retconning it over and over.
as a hero battler, they don't go too deep at first directly.. its like a Streetfighter, etc. just some basic character setups.
Then they established the summoners rift, summoners. Which was a tribunal. take your arguments to the rift, to fight there in a controlled setting, so they don't fight wars, that could annihilate cities with super magic. even the void invasion was trapped in the rift.
then they started an ongoing story. Leblanc/jarvan, demacia/noxus assassinations. swain versus leblanc. and lots and lots of shipping. Cities established at this point, and regions. no map yet.
All that got retconned out. the summoners and the rift are non-canon now. They hired lore writers to set up a world, and started writing new backup stories. been going from there. And, again keep retconning characters backstories. some very heavily. Leona, sivir for example, massively changed. al the Mt. Targon stuff. major changes to Kayle/morgana. Soraka, warwick. leona's was changed like 3 times, as they changed targon lore, and introduced Diana. like, they had leonas story.. then when they introduced diana, they changed Leona's to fit in with Diana, and diana trying to kill Leona.... then they changed their minds on that, and now Diana is a good guy, and leona and diana are in lesbians and working together.
Timeline for alot of this. Arcane went into production when Jinx was created. So, yeah, the story must have been written back then.
Arcane itself, again changed lore. Camille as noted. the timeline for hextech. Vi being her name, when it was clearly a number. Vi's amnesia.
Its kindof interesting they didn't update the lore to match Arcane, in any of the background materials, until Arcane came out. When again.. Arcane was written 8 years ago.