IMPORTANT: This is not a replacement for regular backups. It is intended for use by mod and modpack developers.
Normally, Minecraft will respond to missing blocks/items by replacing them with "air", meaning your test world is corrupted if you accidently removed modded blocks or items.
This mod detects missing blocks (including block/tile entities) or items in your save file and replaces them in game with a surrogate holding all the original data.
When these surrogate blocks/items are saved, the data is written back into the save file exactly as it was read.
So you can fix the problem with the missing/broken mods and recover the data by reloading.
Documentation for additional information.
This mod is for Fabric. Forge has a different mechanism, see the documentation for an explanation.