I was working on a structure based project in 1.21 Pre-Release 1, and realized while saving structures that the "generated" folder inside of the world uses the outdated "structures" directory.

This directory should have been changed to "structure" in 24w21a, alongside the change in the datapack format.

Notes

I had a friend confirm this issue before reporting it, he confirmed that 1.21 Pre-Release saved structures use the plural form directory. Structures saved to the plural form directory also load as intended unless you copy the namespace to a datapack, where the plural form doesn't work, which is the intended behavior.

This error is not present in 24w21b, as the structure block saves to the singular "structure" folder directory.

This issue seems to be caused by MC-272347 where instead of updating old worlds to use the new directory during upgrade, the old directory is used.