I heard in the early versions, there was a feature that a default wallet is automatically created if it does not exist
Yes, those are the earlier versions before 0.21.0. (01-16-2021)Here's the release notes: github.com/bitcoin/bitcoin/blob/master/doc/release-notes/release-notes-0.21.0.md
To get to the repository's state at that time, click your prefered version's tag above "Compare" button in the 'release' page: github.com/bitcoin/bitcoin/releases?page=2
Example for v 0.20.1: github.com/bitcoin/bitcoin/tree/v0.20.1
To elaborate on this, it may be easier to git clone both repos locally, then open them in VSCode/Sublime/Notepad++, etc and run the comparison there first. Once you find the corresponding changes, work backwards and reimplement the feature. It doesn't seem like a big change though, so it shouldn't be too hard. Make sure you have a dev environment set up so you can compile and debug. If you need recommendations or even an OVA.OVF with everything ready to go let me know.
