Minecraft 21w39a
The third 1.18 snapshot is out, and old worlds can finally be opened again. See the changelog.
Drowned Spawns
In dripstone caves, zombies no longer spawn, and drowned can spawn in underwater aquifers. Drowned spawn at the same rate and group size as zombies.
Advancements
A player must be at y level 319 or above while trading to get the Star Trader advancement.
To get the Caves & Cliffs advancement, a player must fall at least 379 blocks, from 319 or above to -59 or below.
Note that the advancement y levels are hardcoded. Custom world heights will not change the advancement conditions.