Existing flat worlds' structure generation is messed up

When you load an existing flat world in 1.21, you get some unexpected structures in new chunks.

Expected result:
The world only generates Villages or Strongholds

Actual result:
The world generates Villages, Strongholds, Trial Chambers, Pillager Outposts, Ruined Portals and Mineshafts, however Mineshafts aren't real, but you can still /locate them.

Steps to reproduce:
1. Create an installation 1.18 in the Launcher > Java Edition > Installations > New Installation > Version section
2. Launch the game
3. Create a normal Flat World with generated structures
4. Exit the game (Save and Quit works)
5. Launch the game now in 1.21 (Newest Snapshot/Release)
6. Load the world
7. Teleport to a position where you weren't at

Existing superflat survival worlds are getting corrupted by these structures, even if the player doesn't need them.