(this should be posted on reddit but due to the damn filter I only can post it here)
Found this when I was playing around with vphone 1337.
only tested on 26.1, so don't have too high expectations for iOS 26.2
ONLY WORKS ON vphone-1337 for now
Steps:
Install Terminal/openssh
Connect to terminal, then run the following commands:
mount -o rw /
cd /System/Library/FeatureFlags/Domain
Open SwiftUI.plist and add the following to the file (under the Solarium section)
<tag>Enabled<tag/>
<false/>
Once done, save the file and reboot. you should see the Old UI now.
Note: some parts are broken.