MC-150020 the composter is now under "Decoration" instead of "Miscellaneous" in the Creative inventory. Apparently the recipe advancement was moved respectively as well:
- 1.15.2: minecraft:recipes/misc/composter
- 20w06a: minecraft:recipes/decorations/composter
However, it appears there is no datafixer for this, causing the following warning message when upgrading old worlds and causing the player to lose the advancement.