Windows 10 wants to delete a previous version of windows. What specfically will be deleted?

Windows 10 recently gave me the following notification:

Delete previous version of Windows?

Your computer's running low on storage space. We can free some up by deleting your previous version of Windows. Warning: you won't be able to go back to it.

What specifically will be deleted if I go through with this? Is it just Windows 8.1 system files? Or will any of my personal files get deleted too? I'm guessing it's the former, but I want to be absolutely sure first, since the process is irreversible.

Just as indicated in the Microsoft post Delete your previous version of Windows, this will delete the "previous version" of Windows you had installed prior to the Windows 10 upgrade.

You're likely getting this notification if it's been 30 days since you completed the Windows 10 upgrade as also indicated in this article.

Lastly, as mentioned by Microsoft in this article, selecting the option to perform this operation will delete the Windows.old folder and prevent you from being able to rollback\downgrade the update to the previous version of Windows before the machine had installed before you completed the Windows 10 upgrade.

Delete your previous version of Windows

One month after you upgrade to Windows 10, your previous version of Windows will be automatically deleted from your PC. However, if you need to free up disk space, and you’re confident that your files and settings are where you want them to be in Windows 10, you can safely delete it yourself. Keep in mind that you'll be deleting your Windows.old folder, which contains files that give you the option to go back to your previous version of Windows. Deleting your previous version of Windows can’t be undone.

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