Andrew Cunningham, Ars Technica, takes a look at look at the state of 64-bit support in the iOS ecosystem.
On the hardware front:
The hardware transition is nearly complete, which is sort of astounding given that the first 64-bit iDevice came out less than two years ago. In all likelihood, the iPhone 5C will fall off the end of the lineup when new iPhones arrive. The iPad is already there.
The two major holdouts are the iPod Touch and the Apple TV.
On the software front:
What a difference a year makes. [30 of the most common apps] are all in active development, and it looks like the submission requirements are working as intended. Gmail is the only 32-bit holdout, which makes sense since it hasn't been updated since March of 2015—one assumes Google's development efforts have mostly been refocused on Inbox.
However, as Andrew points out, there is still the issue of legacy support.