Not exactly. It’s possible to make an app run on older versions (select lower minSdk) and yet take advantage of newer features (select higher targetSdk). This one can be easily be fixed by the F-Droid developers like almost all other developers have already.
Droidify is amazing, I’d totally recommend that over the official F-Droid client any time. 👍
Disappearing icons is a bug with your launcher, nothing to do with the actual browser you’re using.
If you’re looking for a browser that was specifically designed for web apps, check out Hermit. (Disclaimer: I’m the author).