Here is a bash script that implements the desired functionality:

#!/bin/bash

# Specify the extension of the files to be processed
extension=".txt"

# Loop through all files in the current directory and its subdirectories
for file in $(find . -name "*$extension")
do
  # Extract the date that the file was last modified
  date=$(date -r "$file" +%Y-%m-%d)

  # Create a directory with the name of the date, if it doesn't already exist
  if [ ! -d "$date" ]; then
    mkdir "$date"
  fi

  # Move the file into the directory named after its modification date
  mv "$file" "$date"
done

This script uses the find command to locate all files with the specified extension (.txt in this case) in the current directory and its subdirectories. The date command is used to extract the modification date of each file, and the mkdir command is used to create a directory with the same name as the modification date if it doesn't already exist. Finally, the mv command is used to move the file into the directory named after its modification date.
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