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 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 (or Build Number) of the version you want to downgrade. You can get the Build Number Version of most apps on AppStore from Tool Lantency.

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 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 follow 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.: 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 is 826067593? - Read Preparing Section carefully!

Step 5. Install the IPA.

The IPA will be saved at C:\Users\<User>\Music\iTunes\iTunes Media\Mobile Applications. Tips: Open the IPA in Explorer by going to iTunes => Library => Right-click on the app => Open in Explorer

Windows: It is a pain in the neck to install the IPA via iTunes. We will avoid iTunes. 3uTools & iMazing are good alternatives.

3uTools: Drag & Drop the IPA into the Apps tab

iMazing: Manage Apps => Device Tab => Drag & Drop the IPA into the window. (Note: iMazing does work on Windows, but I'm too lazy to setup iMazing on Windows just for 2 screenshots)

macOS: just Airdrop the IPA from your Mac to your phone. You can use iMazing as well.

IV. FAQ

1. Can Apple revoke my IPA?

No, they can't! If you use iOS long enough, I'm sure you remember the time when we can download IPA directly from iTunes. Our IPAs came from iTunes just like an app from AppStore. Apple can do nothing about it.

2. I got errors installing the IPA via AltStore/Sideloadly/...

Again, the IPA is encrypted with your purchased ID. DO NOT sideload it. I mentioned how to install the ipa in Step 4. (Keep in mind that you only sideload decrypted IPA!)

3. What happens if I use another Apple ID?

You have to re-start the process from Step 3.

4. Can I share the IPA with my friends?

Absolutely yes! However, you have to give them the IPA with the Apple ID you used to get the IPA. Why? Because the first time they open your app, they will be asked to enter the purchased ID. That's how iOS works.

Don't want to share your main account? Just use a clone Apple ID.

5. Is it possible to install the IPA without a computer?

Yes, it is possible! However, it requires more additional Steps. I may add it in the future.

README.md

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

Table of contents

I. Notes