Groups like Windows 10 Mobile Enthusiasts often host backed-up XAP files in their file sections.
Always ensure you are downloading version 8.0.x or higher, as earlier versions may have broken certificates that prevent installation. How to Install Opera Mini XAP on Windows 10 Mobile opera mini xap for windows 10 mobile link
Since Windows 10 Mobile primarily uses .Appx files, installing older .Xap files requires a PC: Download the on your PC. Connect your phone via USB. Open the Application Deployment tool. Browse for your downloaded Opera Mini XAP and click Deploy . Step 3: Direct Installation (The Interop Way) Groups like Windows 10 Mobile Enthusiasts often host
Community-maintained "alternative stores." opera mini xap for windows 10 mobile link