A flatpak runtime contains the basic dependencies that an application needs. It is shared by applications so that application authors don’t have to bother with complicated low-level dependencies, but also so that these dependencies can be shared and get shared updates.
Most flatpaks these days use the freedesktop runtime or one of its derivates (like the Gnome and KDE runtimes). Historically, these have been using the 1.6 version of the freedesktop runtime which is based on Yocto.
The 1.6 runtime has served its place to kickstart flatpak and flathub well, but it is getting quite long in the tooth. We still fix security issues in it now and then, but it is not seeing a lot of maintenance recently. Additionally, not a lot of people know enough yocto to work on it, so we were never able to build a larger community around it.
However, earlier this summer a complete reimplementation, version 18.08, was announced, and starting with version 3.30 the Gnome runtime is now based on it as well, with a KDE version is in the works. This runtime is based on BuildStream, making it much easier to work with, which has resulted in a much larger team working on this runtime. Partly this is due to the awesome fact that Codethink has several people paid to work on this, but there are also lots of community support.
The result is a better supported, easier to maintain runtime with more modern content. What we need to do now is to phase out the old runtime and start using the new one in apps.
So, this is a call to action!
Anyone who maintains a flatpak application, especially on flathub, please try to move to a runtime based on 18.08. And if you have any problems, please report them to the upstream freedesktop-sdk project.