It is confirmed: Windows Phone 7.8 won't receive mainstream support after September 9
Luckily, Windows Phone 7.8 will receive the so-called Extended Support over the course of the following 5 years – this means that security updates and hotfixes will still be pushed to devices that rock WP 7.8 for free. During said time period, Microsoft won't accept any requests for design changes for the platform. Hopefully, most users will have moved on to devices that run an up-to-date version of Windows Phone by that time.
Windows Phone 7.8 was the last update for Windows Phone 7 devices, which arrived early 2013 for supported devices. It brought a number of novel features and functionalities to the table - it came with resizable Live Tiles and allowed users to choose between small, medium, or large ones. In addition, the update introduced a beefed up palette with accent colors, which allowed deeper customiziation of Windows Phone 7.8's interface. Initially, the update caused some issues, but these were resolved after a relatively short one-month break.
Some of the more popular devices that supported Windows Phone 7.8 were the Nokia Lumia 510, Lumia 610, Lumia 710, Lumia 800, and the Lumia 900. Are you still sporting one of these?
source: Microsoft Support via Neowin