Anyone trying to do wireless with low power should be using other protocols. Zigbee, Bluetooth, Z-Wave, ANT... anything but WiFi. I mean, WiFi just wasn't designed for low-power like these other protocols.
Hell, you might do better with LoRa (long-range, ~3-miles) signals than WiFi.
> Other ESP8266 dev boards (e.g. D1 Mini, NodeMCU) do support waking up from Deep Sleep. But these dev boards often contain a voltage regulator which consumes a quiescent current. On my D1 Mini boards, I get about 0.22 mA in Forced Deep Sleep mode. In theory, that's enough for 1 year of life on a single charge of a 2000 mAh battery. But waking it up every 15 minutes, for 5-6 seconds of WiFi time, reduces the battery life to about 4-5 months. At this point, the active power usage during the 5-6 seconds of WiFi is already 2X the power consumption of the 15 minutes of Deep Sleep, so I have not attempted to reduce the idle consumption any further.
Have you tried going even slower? You can cut your current usage in half by going every 30-minutes, and then in a quarter by connecting every 60-minutes (etc. etc.) until your current consumption is as low as you want.
There are practical limits (is one connection per day sufficient?), but 15 minutes still has some leeway depending on the application.
If you need low-power but still wireless and more updates than that... having a faster-to-initialize and lower power usage to boot (10mA for those 30ms, rather than for 100mA for 2000mS), you'll grossly cut down your power consumptions.