I found the following comments at
https://wiki.yoctoproject.org/wiki/FAQ#What_is_the_release_cycle_of_the_Yoc
to_Project.3F
(It does seem like we have been supplying maintenance releases for two
releases back, not just one. So maybe this wiki needs to be updated)



What is the release cycle of the Yocto Project?

Each release of the Yocto Project is subject to its own release schedule
according to the community-maintained Project Planning Guide
<https://wiki.yoctoproject.org/wiki/Planning#Yocto>. It is generally
expected that a new version of the Yocto Project will be released every
six months.

What is the overall support plan for the Yocto Project?

Security patches and critical bug fixes are supplied one release back. No
toolchain or kernel changes are allowed for these updates. Support for
longer periods of time can be supplied by commercial OSVs.