If you’ve installed an unpacked extension via developer mode in Chromium-based Edge browser, you should know this: like Google Chrome, Edge browser nudges on every startup with a message to disable developer mode extensions: “Extensions running in developer mode can harm your computer. If you’re not a developer, you should turn off any extensions running in developer mode”.

How to install third-party extensions manually in Chromium Edge using Developer mode
1. Launch Microsoft Chromium based Edge browser
2. Click on menu > Extensions
3. Turn on developer mode,

4. Click on “Load Unpacked”, browse and select the extension folder you’ve readied before, the extension will be installed.
Chromium Edge asks to disable extensions running in developer mode
If you’ve installed extensions in Edge in the above fashion, when you launch the browser, next time at every startup you’ll be nudged to “disable developer mode extensions”, that’s the behavior of Chrome, which Edge is exhibiting now.
At present, there is no way to prevent or turn off this annoyance in Chrome and that’s the case with Edge also. Do note, Google has put this safety measure to prevent users from installing malicious extensions from outside web store on their machines.
Note: new Edge allows to install extensions from Chrome Web store by toggling a setting in edge://extensions, but Google or Microsoft doesn’t recommend installing extensions manually via developer mode.
If you’ve installed third-party extensions manually in Spartan Edge by turning on “Enable extension developer features” flag in about:flags page, Edge browser on its every launch turns them off and shows a message “we’ve turned off extensions from unknown sources” and allows you to run the extension(s) for single time by providing option in message.