This action can be run from Drafts itself to search the “all” tab, or can be run from Shortcuts, meaning you can modify it to run from the widget, or to accept voice input, or put a home screen icon for it or whatever.
https://www.icloud.com/shortcuts/7a401a215fbe475ea7f176e6bec067b9
Of course, Spotlight search on iOS should be doing the same thing. However, I find my spotlight search sometimes decides to stop showing Drafts content (and content from other apps as well). This is more consistent for me.