I changed wallpaper on my iPhone SE running IOS 15.2, then deiced I didn’t like it and tried to change back. I couldn’t.
I went to Settings => Wallpaper, There’s an image of the original wallpaper and the current wallpaper. If I choose the original wallpaper, my options are Cancel, Perspective Zoom, and Set. I’m guessing that the original image is the Lock screen and the other image will all of the icons is the home screen. So that experiment didn’t change the home screen wallpaper back.
If I go to Choose a New Wallpaper, as I did when I originally changed it, I can click on Dynamic, Stills, or my own photos. One would think the original image would be in Stills. It isn’t.
I assume if I do a Reset, the original image would return. But my track record on resetting isn’t great. (Recall my inability to restore my Mac from backup a while ago.) I don’t want to lose any of the apps I’ve added, nor anything saved on the phone. I’d also prefer not to have to move icons around.
My understanding is that the old wallpaper is part of the old iOS. If you switch to a different wallpaper, you no longer have the option of going back unless you proactively saved it. [ETA: But on reading the last reply, it sounds like it may still in fact be saved on your phone, so you may be fine.]
Downgrading to older versions of iOS is not supported by Apple, and will likely screw up your phone. A factory reset/restore does not downgrade the OS. Instead, it simply erases the information and settings on your iPhone and installs the latest version of iOS, which is the opposite of what you want.
If you do a Google search, you can find old wallpapers from previous versions of iOS that you can download. I’ve found these before.
Interestingly, if you never change your wallpaper, when you upgrade you do still keep it, and it seems to work even on devices with larger screens. I’ve got the same wallpaper on my iPhone 11 that I was using on an iPhone 4 from over a decade ago.
Just curious, why would you even want to go back to the “original” wallpaper? Take a picture you like and use that. Or you can snap a screen grab pic of the “default” wallpaper (assuming you can still see it on your phone) and then use that pic.