Wow! I wonder what apps are so great they got abandoned! And no replacements where made......
You would be surprised...
I have a really great study app for Kanji which has no replacement at all. I would happily pay a lot of money to be able to keep it. It's 32 bit, only had one release and no need for updates as it was (and still is, perfect). It's an SRS app, but instead of being a flashcard or pick from multiple choices, you actually have to type in the hiragana reading for the kanji. It is excellent for memorising (although I'm getting old so need to keep refreshing my memory as things don't stay in my brain for long these days) as it has both visual and physical components.
There is NO alternative to it. I've been searching the app store since I was first made aware of this on MacRumors. The developer isn't contactable. I wrote a plea on the review page, but of course they wouldn't have seen it. There is nothing I can do. I would rather have access to this app than a new phone, so I'll be keeping my lovely 7+ and trying to stave off updates for as long as possible.
That probably fits your definition of sad, but some people really do need certain apps in their lives. Unfortunately a lot of us seem to be in very small niche markets where no one else feels the urge to make replacements/alternatives so we're stuck!