If you have ever used Good Lock's Themes Park module before, creating some of your own icon styles before upgrading to OneUI 7, there is an option to restore the old icon layout.

Open Themes Park, open Icon style tab, and tap an old icon pack which you created only using Themes Park.

Tap edit, it will open the icon pack editor and you can then undo all customizations (icon color, frame color, and frame itself) by tapping the x at the top of the color and frame selection. The icons will revert to their original OneUI 6.1 look, and even older looks, if you changed them on OneUI 5. Save your icon pack and apply.

Enjoy in your (new) old icon style 😎 

OneUI 6.1. icon style