You can technically download the iOS 12.1 IPSW file for your iPhone 8 (GSM) Go to product viewer dialog for this item.
You cannot downgrade from a newer version (like iOS 16) to 12.1. 📥 How to Download (Archive Only) Download iOS 12.1 for iPhone 8 (GSM)
If you need the file for archival or advanced research purposes, you can find it on community-driven sites: Go to the iPhone 8 (GSM) page on IPSW.me . Identifier: Ensure you choose iPhone10,4 (the GSM model). Build Number: The specific build for iOS 12.1 is 16B92 . 🛠️ Official Installation Method (Latest Available) You can technically download the iOS 12