While there are some I'm still working on, we have a bunch of dead apps. Things that haven't been updated in years, and no plans to ever do more. (With the new day job, even the actively developed ones get changes and updates much more slowly.)
We started the TopAppCharts.com website back in July 2009. It doesn't have every app, but has all those that got into any of the US top 200 lists for any category since then. The database says there are over 410,000 apps. That's an interesting stat right there, there's way over a million apps, so well over half of the apps released never got into any US top list and were likely dead on arrival.
Apple's App Lookup does not include the "last updated" date, and I wanted to use Apple's public APIs and not scrape the iTunes site, so I had to look at the list of supported devices. Any app that was compiled to support a newer iOS and take advantage of its new features (like those new-fangled Retina screens back in 2010) would lose support for older devices, which gives a rough idea of when the app last had any updates.
Original iPhone support ended with the release of iOS 4; June 2010
iPhone 3G support ended with the release of iOS 5; October 2011
iPhone 3GS support ended with the release of iOS 7; September 2013 (iOS7 required for submission Feb 2014)
iPhone 4 support ended with the release of iOS 8; September 2014
I can't find dates for some older things, when iOS 4/5/6 support was required for submission.
38,362 or 9% of the apps we know about still support the original iPhone, so haven't been updated since around 2010 and wouldn't support Retina screens.
20,320 or 4% still support the iPhone3G, so haven't been updated since 2011.
137,027 or 33% still support the iPhone 3GS, so haven't been updated since 2014. These would look old-style on iOS 7 and definitely wouldn't take advantage of iPhone 6/6plus screens.
So 46% of all the apps tracked by our site haven't been updated since last year at the earliest. And 13% haven't been updated in 3+ years. The huge number of apps we don't have in the database never got into any top selling list since 2009, so I'm sure would be far higher percentage of "dead".
There's a good chance those old apps will have some problem with newer iOS versions, so probably won't work on your new iPhone 6 anyway.