Linux 5.1 released has just been released. The main feature in this release is io_uring, a high-performance interface for asynchronous I/O. There are also improvements in fanotify to provide a scalable way of watching changes on large file systems, and it adds a method to allow safe delivery of signals in presence of PID reuse. Persistent memory can be used now as hot-plugabble RAM, Zstd compression levels have been made configurable in Btrfs, and there is a new cpuidle governor that makes better power management decisions than the menu governor.
In addition, all 32 bit architectures have added the necessary syscalls to deal with the y2038 problem; and live patching has added support for creating cumulative patches.
There are many other features and new drivers in the KernelNewbies changelog.