Before 1.18, ocean biomes had seagrass that generated underground in underwater caves. In 1.18+ this underground seagrass no longer generates, even though the files for the feature still exist.

Reproducing the Issue

Create a world in a version before 1.18. I used 1.17.1. Find an ocean and teleport to it. Observe that seagrass generates in underwater caves below the sea floor.

Create a second world in a version after 1.18. I used 1.19.4. Find an ocean and teleport to it. Observe that seagrass does not generate in underwater caves below the sea floor.

The Cause

The root of this issue is in the first placement modifier of the seagrass_simple placed feature file.

{
  "type": "minecraft:carving_mask",
  "step": "liquid"
} 

This placement modifier is supposed to attempt to place the feature on every block carved by a carver in the liquid step. However, the liquid step is unused in 1.18+ as aquifers replace them.

The solution would probably be to generate them similarly to how other underground features generate, with many placements per chunk and with environment scans to put them on underwater cave floors.