I see at KernelVersions master being at 5.4.
I've no idea what are openwrt's dev cycles but ... It would be pretty neat to get v5.10 because of the recent improvements at mt76 driver, and possible other reasons.
I don't know the details, but it's probably true. It seems that patches submitted to the ML of the Linux Kernel and not yet merged are being actively tested in the repository of OpenWrt.