Limiting "Changed" output or optimizing playbook output

Here's a bit of code for finding files based on some criteria (in this example, files greater >=40k).

---
- hosts: nodes
  become: true

  vars_files: my_vars.yml

  vars:
    where: "/etc"
    what: "*.conf"

  tasks:

    - name: Find all files greater than 40k
      ansible.builtin.find:
        paths: "{{where}}"
        size: 40k
        recurse: yes
      register: output

    - name: Output the above
      ansible.builtin.shell:
        "echo {{item.path}} >> testfile3" 
      loop: "{{output['files']|flatten(1)}}"

Now, this runs "successfully" but it throws up a bunch of "changed: [host1] => ..." orange text and the playbook output is a wall of text. Is there a proper way of controlling and limit the output? Or is there a better way to code the playbook?

Based on what I guess you're looking for (please make sure you read How to ask a gook question paying a special attention to the Minimal reproducible example section for your next post), the following should approach your (guessed) requirement:

    - name: Find all files greater than 40k
      ansible.builtin.find:
        paths: "{{where}}"
        size: 40k
        recurse: yes
      register: output

    - name: "Print the paths to a text file on the target node 
            (replacing previous output) from the above register"
      ansible.builtin.copy:
        content: "{{ output.files | map(attribute='path') | join('\n') }}"
        dest: /path/to/testfile3

    - name: If you're only looking for an output, just debug the list
      ansible..builtin.debug:
        var: output.files | map(attribute='path')

To go a bit further and address a few concerns raised in your post:

1. You should not use ansible.builtin.shell when there's a module already doing the job you expect.
2. If you still need to use ansible.builtin.shell, as already raised by @4snok, it is not idempotent by default. You will have to manage idempotency yourself. See Defining changed
3. The base problem in your original playbook is (I guess) the output of loop (which is definitely not needed here). If for some reason you still need a loop in your final code, there are several ways to limit its output:
   • Limit the information your are looping on from start so that you only have the needed data in the loop. As an example:

     - name: Loop on files path in registered result
       ansible.builtin.debug:
         var: item
       loop: "{{ output.files | map(attribute='path') }}"
     

   • Limit the ouput in each loop to only what you want to keep. See Limiting loop ouput with label for more information. Example:

     - name: Limit loop output
       ansible.builtin.debug:
         msg: "Checksum of {{ item.path }} is {{ item.checksum }}"
       loop: "{{ output.files }}"
       loop_control:
         label: "{{ item.path }}"
     

The shell module isn't idempotent. When performing it multiple times doesn't necessarily produce the same result every time it is performed. In your case the content of testfile3 file will be changed every time you run the playbook.
You can control this behavior with changed_when option.

To suppress output from the task use 'no_log: true'.

  tasks:

    - name: Find all files greater than 40k
      ansible.builtin.find:
        paths: "{{where}}"
        size: 40k
        recurse: yes
      register: output
      no_log: true


    - name: Output the above
      ansible.builtin.shell:
        "echo {{item.path}} >> testfile3" 
      loop: "{{output['files']|flatten(1)}}"
      changed_when: false