It’s alive. https://udm14.com
You're welcome, but it's next to nothing, and it doesn't work on Android, but you can modify the Startpage instructions at https://support.startpage.com/hc/en-us/articles/4520868432788-Make-Startpage-your-default-search-engine-on-Firefox-mobile-Android to add one there.
It feels odd that it's not possible to create an Android addon that does this because the overrides are not supported there, but you can follow simple configuration steps to get the same result. Meanwhile, on Windows, you can't make a configured search engine the default. So the solution is different in each case.