The string-disabling mechanic was patched in versions 1.21.2/3, so try using an earlier version.
This farm does not work on Bedrock Edition.
The string-disabling mechanic doesn’t work on Forge loader; it works on vanilla and Fabric. Not sure about NeoForge and Quilt.
Stats (per in-game day): ~ 3+ Stacks of String
Dig a hole in the ground, 4 blocks wide, 1 block deep, 3 blocks long.
2. Add chests vertically and attach hoppers to each chest.
3. Add 3 of your solid blocks on each side of the hoppers like in the picture below and cover the behind of the hoppers, on top of the chests, with glass.
4. Connect the 2 walls in a U-shape like in the image below.
5. Add 2 of your temporary blocks like in the image and attach 2 trap doors on the temporary blocks.
6. Break the temporary blocks and insert water in between the trapdoors and wall so that every item that gets dropped inside gets moved into the hoppers.
7. On one of the trapdoors, place your last solid block and attach a lever on the block above the other trapdoor to be able to control both trapdoors.
8. Infront of the trapdoors, add 2 tripwire hooks on each wall and connect them with 2 strings.
8. Break the string WITH the shears(IMPORTAND) and then quickly pull the lever to open the trapdoors to let water flow and break the strings, then close the trapdoor and open it again and it should start glitching and dropping strings.
8. Now, you should have a fully working string farm.