I think the search function is working fine, the app is just called "AnySoftKeyboard 3D Theme" in the F-Droid app but not on the website for some reason.
With all default repos + IzzyOnDroid enabled and searching exactly for that package name I get all the language packs and a 7 years old PC theme, but not the app itself. I'll just download the APK, I guess.
It shows up for me on GrapheneOS. I don't think the F-Droid search is OS-dependent anyways besides maybe the major Android version number. As the GP noted, for some reason the name is listed with the "3D Theme" addendum in the app which is a little bizarre. Does your browser support opening the link in an app, i.e. F-Droid?