Windows 10 Mobile (version 1703) mainstream support ends in June 2019
Microsoft lists 6/11/2019 as a mainstream support end date
The current public version of Windows 10 Mobile is 1703, released as a Creators Update with build 15063 in April 2017, and Microsoft will support this version until June 2019. A Microsoft Support page lists 6/11/2019 as the end date for the mainstream support, which means that phones like the Lumia 950 and 650 can expect cumulative updates with improvements in the next 20 months.
You should know that only devices running the official public version (1703 / build 15063.xxx) will be supported till mid-2019, while phones like the Lumia 930 are already out of support.
Microsoft exec confirmed last week that Windows 10 Mobile will receive monthly patches in the foreseeable future, but the company is not planning new features for the current generation of mobile devices and Windows 10 Mobile.
Microsoft is expected to reboot its mobile strategy in 2018 by introducing the Project Andromeda – a modular-based Windows with unified core, and a new mobile-type device.