The bug

1.18 aims to simplify world generation in many aspects by deleting biomes which are only trivially different from other biomes, and whose roles in the biome generation system from 1.7 to 1.17 are made redundant by 1.18's terrain generation handling changes. Very many of these were removed and merged into their "parent" biome in 21w40a, with the deep warm ocean biome also being removed in 21w43a.

The deep ocean appears to be a straggler in this sense, since its main purpose was just to make oceans be deeper in the areas where it generated. As of 1.18, this no longer happens, and deep oceans generate terrain no different from normal minecraft:ocean biomes, with the only thing setting them apart being that deep oceans can generate ocean monuments, a functionality that could easily just be moved over to the normal ocean biome.

I've only tested the normal versions of deep oceans so far - this very likely also applies to the deep variants of lukewarm, cold and frozen oceans as well, however.

How to reproduce

The screenshots attached to this issue are of single biome worlds generated with an identical world seed, seen from the same position and angle by using the command
/tp @s 176 76 398 -135 20
to have a constant viewpoint such that the only difference is the biome. It can be clearly seen that all that differs between the two screenshots is the presence of an ocean monument, as well as terrain features which were disturbed by the presence of the adjacent ocean monument and were forced to generate differently to accommodate for it. Everything else generation-wise is the same.

Expected results

Deep ocean biomes would be removed and merged with normal oceans, and ocean monuments would gain the ability to generate in normal oceans.

Actual results

Deep ocean biomes still exist alongside normal ocean biomes, despite having only extremely minor differences.

Further notes

Henrik Kniberg has noted on Twitter that these biomes may be merged in future, but have not been yet presumably due to time constraints: https://twitter.com/henrikkniberg/status/1453411253603426304