Apple - Why is iOS unable to find my apps?
If you are unable to search for apps you could once find, check your settings to see what apps you prevented Search from searching within.
Apple recently changed the behavior of the relevant setting (Siri & Search). Prior to iOS 11 disabling this setting for an app prevented Search from showing results from "inside" the app (e.g. Dropbox listing files). With iOS 11 this behavior has changed: now this setting must be enabled for the app itself to appear in Search results at all.
Note that while this appeared to have been "fixed" in the betas of 11.2 with a new explicit option, "Show App" to search for the app, this seems to have been released (in 11.2.1) as "Suggestions in Look Up" which doesn't achieve the same result.
Today I decided to give iOS 11.2 beta 4 a try. Though I hadn't read about this specific change, I was thrilled to see that Apple has addressed this issue! There is now a separate 'Show App' setting available on a per-app basis. It's still going to be tedious to go in to each of my 300-or-so apps to enable this, but I'm glad there's a way to again use Spotlight to launch apps.
My version of this question: Fixing app search in iOS 11