In 2015 there were just 436 commits for the entire year -- in 2014 there were 923 commits, which was about average recently. The high point of X.Org Server's development was around 2008 when there were 2,114 commits in a single year. Except for 2015, every year saw close to a thousand or more commits going back a decade. In 2004 there were 590 commits while in 2003 there were 125 commits. Of course, prior to 2004 the X.Org Server was technically the XFree86 code-base.
For those thinking that the fewer commits may be due to larger commits, that is not the case either. The 400+ commits last year added just 11,999 lines and removed 9,945 lines... So a net gain of about two thousand lines of code for the year. In comparison, in 2014 there were more than 70k+ insertions and 57k+ deletions. Prior years saw multiple times more code changes than in 2015.
This stall in X.Org Server development can likely be attributed to Wayland/Mir, the maturing of the modern X.Org Server stack, and more focus being placed on shared, external projects like libinput, Pixman, etc. As I shared a few days ago, Mesa saw the most commits ever since 2010 during the span of last year. At the start of 2015 was also when longtime X developer Keith Packard left his post at Intel to join HP where he's no longer working day-to-day on X/graphics. Midway through 2015 is also when Red Hat's Adam Jackson seemed to take over release management roles from Keith.
2015 saw the release of X.Org Server 1.17 at the beginning of the year and then in November was the very belated X.Org Server 1.18 release. Eric Anholt, who has been working at Broadcom and working to improve the X.Org Server for its GLAMOR support, was responsible for 62 of the 436 commits in 2015. Other frequen contributors were Keith Packard, Adam Jackson, Alan Coopersmith, and David Airlie. In total there were 69 listed authors to the X.Org Server in 2015, which is also a low point since 2006.
The X.Org Server Git repository is currently at 707,024 lines across 1,704 files. If you want to dig more into the X.Org Server Git details, see the 2015 data here.