Though dropping support for the Xperia X is understandable (the device as much as I like it is 8 years old by now and as much as I liked it in my older rants it is old), the excuse (old kernel version) is really not a valid one since Sony has released newer kernels for it (and also the newer devices), Jolla has over the years not updated the kernels on devices for whatever reason a policy that is a direct contradiction to the claim to security focus.
(Xperia X has at least kernel 4.14 available to it)