Sure, here's a script that does what you described:

bash

#!/bin/bash

# specify the extension of the files you want to look for
extension=".txt"

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

    # create a new directory named after 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 search for files with the specified extension in the current directory and its subdirectories. For each file found, the script extracts the date it was last modified using the date command, creates a new directory with the same name as the date if it doesn't already exist, and moves the file into that directory.
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