The Guide seems complicated, but you only need to do Step 1,2, and 3 for the first time. There are 5 steps in total.
Jailbreak is NOT needed. Since the iPA comes directly from iTunes, it's encrypted and can be installed without Sideload. The IPA is 100% legit.
You can get the old version of any apps on AppStore as long as that version is still available on AppStore.

II. Requirements

iTunes 12.6.5 - the latest version of iTunes that supports download apps. Download here (directly from Apple).
A Windows machine: Windows XP/7/10 are supported, not tested on Windows 11 yet.

(Why Windows only? - Apple killed iTunes 12.6.5 on macOS. Even if you manage to get iTunes 12.6.5 on your Mac, the download feature will does not work.)

Charles Proxy. I use version 4.2.7 but I don't think it's matter. (No need to crack Charles Proxy)
An Apple ID for iTunes. Use a clone ID if you like. Keep in mind that if you switch to another ID, you'll need to re-do the process from Step 3.

III. Get Started

Step 1. Preparing

Nothing special about Charles Proxy so I'll focus on iTunes.

After you install iTunes succesfully, go to Edit => Preferences => Avanced => Untick Check for new software updates automatically to prevent iTunes from asking for update.

If you get an error about Library.itl when opening iTunes: Go to “C:\Users\Username\My Music\iTunes\” and delete the existed Library.itl

The Build Number Version of the version you want to downgrade. You can get the Build number Version of most apps on AppStore from Tool Lantency. See more info in pictures below:

Step 2. Install Charles Root Certificate

Congrats! You just installed Charles Certificate!

Step 3. Create the Breakpoint

(This is where the fun begins!)

Open iTunes & Charles Proxy

Search for the app you want to downgrade. I will get the IPA of Facebook v161.0 as an example

Select Get or Download to download Facebook. This is not the version we want so we'll delete it.

Now, go to Charles Proxy, we'll see a list of domains. Find a domain that has a form of “p**-buy.itunes.apple.com”, ** is two-random numbers. As you can see in my picture below, mine is “p31-buy.itunes.apple.com”. Right-click on it and select Enable SSL Proxying

Enable the Breakpoint

Go back to iTunes and download Facebook again. This is still not a version we want, so we'll delete it.
In Charles Proxy, you'll see a new p31-buy.itunes.apple.com address with the blue icon at the top of the line. Expand this address to buyProduct. And then following the pictures:

Step 4. Get the old version of the appplication (IPA)

Note: You ONLY need to do the first 3 steps once time. The next time you download an old version of any apps, you'll start from this step (Step 4).
Go back to iTunes and download Facebook, again! Charles Proxy will automatically show the Breakpoint popup. Select Edit Request => XML Text => Replace the current Build Number Version of Facebook with the Build Number of Facebook v161.0: 826067593 => Execute => Execute. Now iTunes will download Facebook v161.0 istead of the latest version.

But how do I know the Build Number of Facebook v161.0? - Read Preparing Section carefully!

How do I know the Build Number of Facebook v161.0? - Read Preparing Section carefully!

Step 5. Install the downloaded IPA.

We've finished the hardest part!👊 The IPA will be saved at C:\Users\<User>\Music\iTunes\iTunes Media\Mobile Applications

Pro Tips: Instead of going to the location above, iTunes => Library => Right-click on the app => Open in Explorer

README.md

[Guide] How to Downgrade apps on AppStore with iTunes 12.6.5 & Charles Proxy (no Jailbreak).

Table of contents

I. Notes