From Nawzil's official post:
Windows 10 Mobile Support Lifecycle
Support for Windows 10 Mobile is almost the same as other editions of Windows 10, but the support duration is slightly longer. Each version of Windows 10 Mobile is supported for around 26 months (2 years and 2 months) from the date of release.
End of support dates for each version of Windows 10 Mobile:
Windows 10 Mobile version 1511 build 10586 / Initial Release: January 9, 2018
Windows 10 Mobile version 1607 build 14393 / Anniversary Update: October 9, 2018
Windows 10 Mobile version 1703 build 15063 / Creators Update: June 11, 2019
Windows 10 Mobile version 1709 build 15254 / Fall Creators Update: December 10, 2019 *
* Tentative dates, support may be extended. The exact date will be announced within 1 year of these dates.
Lumia Support Lifecycle
Lumia phones are supported for 2 years from the date they are launched. This means they get Feature Updates during those 2 years. Each Feature Update is supported for around 26 months. The last Feature Update that a Lumia phone gets at the end of the 2 years will be supported for around 2 years, so the total support period for a Lumia phone is around 4 years from the date it is launched.
In short, a Lumia phone will get Feature Updates for 2 years and it will get Cumulative Updates for the last Feature Update for 2 more years.
We already knew about most of this, but it's great to see the dates so clearly laid out.
Phones then that are FCU-compatible may well be officially supported with fixes and security updates into 2020, which would be quite something! The rest of the mainstream Lumias (I'm thinking Lumia 735, 640, 830, 930, etc.) will end their days on the Anniversary Update (AU), if they've never been part of the Insider programme, or the Creators Update (CU), which rolled out a few months ago and which was obtainable for months (is it still?) via a little Insiders programme 'dance'.
In all cases, phones with each branch installed will receive updates for that branch until the end of support dates listed above.
Right, all clear now?