After several years of playing around with XF (since April 2015, when I bought my first license), I finally migrated my vBulletin 4.2 system to XF 2.1 on March 10, 2019. I migrated to the latest version available, version 2.1.0, which was released in January 2019. Literally three days after migrating, XF released version 2.1.1.
In the exactly 11 months (to the day) since March 10, 2019, we have had XF 2.1.1, 2.1.2, 2.1.3, 2.1.4, 2.1.5, 2.1.6, and now 2.1.7 recently released. Roughly, that's a release about every 5-6 weeks.
While pretty much all of the releases have been bug- and security fixes, I find it difficult to believe that folks feel that XenForo development is slowing down, has less and less activity, and is falling behind.
If you've ever been on vBulletin 4.x .....now THAT's a situation that one can credibly say that development slowed down and fell behind. Actually, it stopped cold in its tracks long about 2013. After 2015, vBulletin 4 was 100% tumbleweeds from a development perspective.
While I DO wish that the XenForo developers would provide a little more of a roadmap -- particularly from a technical standpoint -- into general directions of what is coming, what is being investigated, and so forth, I certainly could never claim that the development has slowed down.