Ruined portals were meant to be spawning by default in superflat as shown in MC-179315 being resolved, but they don't. However, even when I used this preset:

minecraft:bedrock,2*minecraft:dirt,minecraft:grass_block;minecraft:plains;ruined_portal;stronghold(spread=2) 

it still didn't generate any ruined portals, while still generating strongholds.

Update: Ruined portals still spawn in superflat worlds with the unmodified Overworld preset (added two screenshots showing this). They do not spawn in the Classic Flat preset, even when modified to have 120 layers of stone (ruling out MC-241438 being the problem here).