Hello there, how are you doing?
Well actualy it´s pretty easy nowadays to get this kind of information, if you google "Raid Shadow Legends´ promo codes", you´ll see there are many content creators with their own websites which allows everybody to see the active, inactive and expired promo codes. By the way a RSL Series has come to an end a couple weeks ago, try to check that out, you´ll get some juicy rewards from it.
To be more specific, visit Hell Hades and DeadWoodJedi respectively websites, you´ll get advantage from it for sure.
I hope it will help you a bit. Have a great day!!