I specifically remember that Lava Bats and Red Devils would start to spawn when mech bosses were defeated in a pre=Hardmode world. However, I just created a new world to confirm that, and it appears that it is no longer the case. No change after defeating all of the mech bosses & plantera (expected).
After that, I enabled hardmode, and then the Red Devils & Lava Bats started spawning. So yes, apparently it does now require hardmode for the new hell mobs.