Hello jEzEk,
There are both technical and societal reasons as to why we have decided to not accept new features into the Xenial line of update anymore, even the ones which is developed against Xenial and is near ready.
First, the technical reason: due to a large number of changes involve in making all Ubuntu Touch components works with the new version of Ubuntu, compound with desire to realize the rename in this release, we've decided early on to have a separated branch of code for almost every component we have. Unfortunately, this resulted in a largely separated development between Xenial and Focal, and while one is busy making a component runs on Focal, another proceeds to add new features onto the existing codebase on Xenial. And because we didn't realize this problem early enough, this means now we have quite a backlog to visit. Also, since the code diverged a lot since we start the branches for Focal, sometimes it's not straight forward to port changes to Focal. So, limiting the amount of changes that enters Xenial directly reduces the amount of work needed to make Ubuntu Touch based on Focal a reality.
Now, the societal reason: as mentioned by many people above, we're a small group of people and unfortunately can't do everything at once. In addition to adding the amount of changes we need to carry to Focal, time spent reviewing changes for Xenial is time not spent developing or reviewing changes for Focal. Given that Xenial has been out of support for over a year at this point, it has become our priority to provide our users with the up-to-date base for the security of our users, and that means we need to reduce as much work as possible.
In addition, many people might consider Focal complete when it reaches feature parity to Xenial. However, if more changes keeps being added to Xenial, it becomes harder to realize that objective. Having to chase a moving target is not fun, and personally I already have that feeling when I realized that the contact backend changes will land in OTA-24. Fortunately that didn't happen in the end, but even for smaller features it can easily be unexpectedly hard to bring to Focal.
So, please accept my apology that we cannot accept more changes into Xenial line of update anymore. We want to have an up-to-date and secure operating system, and we cannot have that with an out-of-date base.
On the side of device support, the primary limiting factor at the moment is due to the inevitable change of the init system from Upstart to Systemd. Systemd uses a number of modern features of Linux kernel, and as such requires a (moderately) modern Linux kernel. The version of systemd requires Linux >= 3.13, however the recent version raised that a bit.
It's possible to backport certain changes into an older kernel to make systemd runs, but it's a tedious manual task that has to be done per kernel tree [1]. A rule of thumb is that if the device launches with Android 9 it should be fine, but for older devices one must check the Linux kernel version.
[1] For example, I've done such job on FP2, which allows me to run an old image of Plasma Mobile which uses systemd.
