Minecraft wiki entry for Ocean, associated text: Before 1.18, the plains and forest biomes generated in patches within regular deep ocean biomes, similar to how "hills" biomes generated within their respective base biomes, creating islands.
Although normal deep ocean biome could generate standalone, normal deep ocean biomes also generated in patches within normal ocean biomes.
Before 1.18, regular and deep frozen oceans could generate near any kind of biome. It was somewhat common to see an iceberg bordering a desert, savanna or badlands in Bedrock Edition. Until 1.18, frozen oceans had a rough regular ocean border to minimize this effect in Java Edition.