Vanilla 1.21.1 1.0.0 is a release file of the Minecraft resource pack SI: Legacy UI - Lightmode Purple [DISCONTINUED], made for Minecraft 1.21.2-Snapshot, 1.21, 1.21.6-snapshot, 1.21.1-Snapshot, 1.21.5, 1.21.4, 1.21.5-Snapshot, 1.21.1, 1.21.6, 1.21.3, 1.21.7, 1.21-Snapshot and more. It was published on July 3, 2025 and is 0.4 MB in size. For the full description, screenshots and every other version, visit the SI: Legacy UI - Lightmode Purple [DISCONTINUED] resource pack page.
How to install: Put the .zip into .minecraft/resourcepacks (do not extract it) and enable it under Options → Resource Packs.