Xenomai will run v5.10 LTS as soon as release 3.2, based on the ongoing port of the Cobalt core to Dovetail.
How long a particular Xenomai release is maintained for a particular kernel version currently depends on a few individuals and companies having a specific requirement to keep such port alive, and contributing to this task directly (doing the work) or indirectly (funding someone else's work).
Although there is a commitment to run Xenomai 3.x on top of kernel v5.10 asap, I'm not aware of any formal commitment of any current contributor to maintain this port for a pre-defined period of time. One may only assume that by picking a LTS kernel like v5.10, we are likely going to maintain it "for some time".