In the 1.18 update, you can find structures below y=0 such as dungeons and mineshafts. However, strongholds cannot be found below y=0. This is already a feature on Java, so this can be considered a parity issue.
Step to Reproduce
1. Create a new world.
2. Do /locate stronghold and teleport to that stronghold.
3. Check if it's generating below Y=0/on negative Y levels.
4. Repeat this from step 2 but by teleporting away from the previous stronghold.
Some of the strongholds are generating below Y=0/on negative Y levels.
None of them are generating below Y=0/on negative Y levels.